According to Wikipedia, "Bangkok, officially known in Thai as Krung Thep Maha Nakhon and colloquially as Krung Thep, is the capital and most populous city of Thailand."
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Part 1: Understand. Bangkok's cultural heritage dates from 1782, when King Rama I built his new capital in Rattanakosin. In this itinerary, you'll explore this historic island with its famous temples in the morning. When the sun gets at its strongest, it's best to cool down in some air-conditioned malls or indoor attractions. The evening will show you more of Bangkok's modern face, with its bustling and hip nightlife scene. Note that the Grand Palace, Bangkok's most popular attraction, is purposely not included in this itinerary. As the Palace is located right next to Wat Pho, you can visit it instead or even try to squeeze in both if you wish, but the palace grounds are so large that it's probably best to visit it on a separate day.
Part 2: Prepare. Bangkok is relatively close to the equator, so do not underestimate the heat. Start out early if you can, because temple-tramping during the afternoon heat is draining and there won't be much air conditioning along the way. Drink enough fluids, but don't carry huge bottles of water in your backpack, as there are plenty of convenience stores selling cooled beverages. As you will be walking most of the time, wear comfortable shoes and dress for the climate. Don't go overboard though, pants must have long sleeves and shoes must be neat, as temples and neat restaurants have strict dress codes. There are surprisingly few (good) places to eat in Rattanakosin, so pack in a good breakfast before you go. There are two parts to this itinerary: a morning of canal cruises and tramping through temples, and two choices for an evening of entertainment, fine food and drinks. As you will be criss-crossing through the city the whole day, get familiar with Bangkok's public transit system. The BTS Skytrain and MRT Metro are the easiest ways to move around Sukhumvit and Silom. Getting to Rattanakosin can best be done over water, with the Chao Phraya Express Boat and the Saen Saep Express Boat. You can also pullover a metered taxi. If you keep an eye on your budget, the day won't cost more than 1,000 baht, unless you visit Siam Ocean World, which by itself is already a whopping 990 baht (discounts available online). Entry of temples and short taxi rides are not more than 50 baht, while mid-range lunch and dinner are around 250 baht each. If you are really on a budget, you might want to eat at street restaurants for not more than 50 baht a meal.
Part 3: Itinerary
If your accommodation is around Sukhumvit or Silom, your starting point will be Ratchathewi station (N1), easily reached from the Skytrain Sukhumvit line. Take exit 1 and head a few hundred metres south (after walking down the stairs, turn 180 degrees and walk down the street) and walk over the bridge across the Saen Saeb canal. To your right, you will see the small, rickety pier called Saphan Hua Chang. Board the next canal boat that comes barreling down towards the west (make sure the boat is heading left as you stand on the pier, from under the bridge) — but move quickly, because these boats don't wait for lingerers! Pay your fare (9 baht) and enjoy a view into the backyards of Bangkok. Some points of minor interest that you will pass along the way include:
Jim Thompson House, an attraction in itself and worth a visit if you have some spare energy later;
Bobae Market, one of Bangkok's largest and cheapest clothing markets (not many tourists around here!).
Disembark at the Phan Fa Lilat terminus near the Golden Mount. If your accommodation is around Khao San Road, you can just get here on foot. To your left, you'll see a small bridge with oncoming traffic, and a large bridge with multi-lane traffic. Cross the large bridge, pass the white Mahakan Fort and cross the street for Wat Ratchanaddaram, one of Bangkok's unappreciated temples. Entry is free (although a recommended donation of 20 baht is requested), just take your shoes off and climb into the tower for a bird's-eye view of Rattanakosin.
Once finished, head back to the main street and hail a metered taxi (not a tuk-tuk) for a quick trip to Wat Pho; if you insist on the meter, this won't cost you more than about 50 baht. Entry into Wat Pho will set you back 200 baht, with a free bottle of water included. The world's largest Reclining Buddha is in the perennially busy building to your right as you enter from the northern entrance. For good luck, you can do as the Thais do and buy a bag of 25-satang coins (20 baht) to plink, plink, plink into the pots behind the Buddha. Explore the rest of the large complex, and if you feel like taking a break, pop into the famous massage school at the eastern side of the temple grounds (from 320 baht for 1/2 hour).
When done, exit through the west gates (behind the Reclining Buddha) and look for signs pointing towards the pier of Tha Tien, which you will find half-hidden past a market alley full of stalls selling dried squid and such. You will see the white spires of Wat Arun (Temple of Dawn) across the Chao Phraya river. Hop aboard the angular little ferry and pay 4 baht (each way) for the trip. Seeing Wat Arun closer from the gardens around it is free, although quite frankly, it looks better from a distance. You will be charged 50 baht if you want to climb up (increasing to 100 baht on June 1, 2020). Note how the decorations of the spires are in fact all made from millions of bits of smashed white porcelain painted with patterns! Head back across the river on the ferry you came on and cross the pier to the Chao Phraya Express Boat dock, clearly identified with blue-and-white signs. As of April 2018, there are maintenance works for the express boat pier on the Wat Pho side as indicated on signs on the pier, so boats to both directions are leaving from the Wat Arun side, removing the need to cross the river again. Head to Pier 2 for southbound (downriver) boats, wave down the next downriver express boat and ride down all the way to Oriental Pier (N1) for 13 baht. From there, get to the Mandarin Oriental, often judged as the best hotel in the world. Just walk down the alley leading from the dock and turn left. Reward yourself with a drink at the famous Bamboo Bar, although (alas!) live jazz is played only in the evenings. It's expensive though: a beer will cost you a whopping 250 baht. You won't get in if you are wearing sandals either.
From the Oriental, head for the Saphan Taksin BTS station (S6) by either taking the express boat one more stop downriver to Sathorn Pier (Central) or by walking out onto Charoen Krung Road, turning right and walking some 15 minutes south.
Take a breather from the sweltering heat at the hotel swimming pool... or, if you still have energy to burn, go shop in any of the air-conditioned shopping meccas in Siam Square. Teens will love MBK Center and women will love Siam Paragon and Emporium, at least as long as their credit limits allow. Families may like to visit the Siam Ocean World in Siam Paragon, which sets back a whopping 900 baht for adults and 700 baht for children. All attractions can be found at the Siam Skytrain station, except Emporium, which is headed at the Phrom Phong Skytrain station in Sukhumvit.
If you're up for more cultural sights, you might want to visit Jim Thompson's House, which you've seen earlier when boating your way through the canal. The former residence of the American expat Jim Thompson is a kind of ideal-house in Thai-style, and a nice escape from the traffic chaos elsewhere in downtown. Entry is 100 baht and it is easily accessed by the National Stadium Skytrain station. From there, walk 200 metres north into Soi Kasemsan 2.
Two alternative routes here: either the traditional Bangkok of teak houses and go-go bars, or the brash new Bangkok of super hip restaurants and clubs.
Dine at Baan Khanitha, a fair hike down Sukhumvit Soi 23 from Skytrain Asok (E4). Excellent Thai food, although the amount of chili is toned down for tourists (just ask, and they'll spice it up if you're brave enough to try!). Particular favourites here are the pomelo salad (ยำส้มโอ yam som oh) and roast duck in red curry (แกงแดงเป็ด kaeng daeng phet). Mains mostly range around 300 baht. Hop on the Skytrain and change trains at Siam for Sala Daeng station (S2). Follow the crowds a few hundred metres down Silom Road and through the street market until you spot the signs of Patpong to your right. Pick a go-go bar of your liking, perhaps King's Castle III if you're adventurous and want to see transvestites strutting their stuff. Beers around 70 baht a pop.
Dine at Long Table, a fair hike down Sukhumvit Soi 16 from Skytrain Asok (E4). A super slick lounge in scarlet where you will lie on futons and dine at one long table on a multi-course meal of Thai inspired fare with a twist, complete with breathtaking views of Bangkok's cityscape. Very expensive by Bangkok standards though, so expect to pay around 1,500 baht per head with drinks. Reservations are recommended. Feel free to stick around for an active nightlife all days of the week.
Take a taxi to the State Tower at the western edge of Silom Road, easily spotted thanks to its golden dome, and then ride the elevator to the 63rd floor for the Sirocco and Sky Bar, the world's tallest rooftop bar/restaurant with mind-boggling nighttime views of Bangkok below. Drinks are 400 baht and up, with a live jazz band for free. Take note that both of these have a dress code: no shorts or sandals.
4. Stay safe
Touts can be a hassle at many of the better-known tourist sights. Don't trust a single word of unsolicited advice, especially if it involves temples closed for Buddhist holidays, 20-baht tuk-tuk rides and/or their cousin's jewellery shop. Don't let strangers steer you away from your intended destination.
When taking a taxi, be sure to insist on the meter. If the driver refuses, or quotes silly prices, just walk out and get a different one, as usually there are plenty of them available. If multiple taxis refuse to take you, or you just want to ride it once, you can take a tuk-tuk, but always agree on a price in advance.
In the go-go bar zones, beware of touts who try to drag you into the upstairs bars with offers of ping-pong shows and 100-baht beer. The beer may well be 100 baht, but the "show" you'll be treated to will be 1,000 baht or more. Rule of thumb is, if you can't see inside from street level, the establishment is best avoided.