Collections can be quite fun. Everybody can have some kind of collections and pass to the next generation, which could be more fun. You can collect what you love, but it must be affordable and easy to store.
The most common collections include stamps, coins, paper money and jewelry. Those collections cost some money, but can worth more in the future.
There are also some popular collections that could take lots of space, such as books, toys and furniture.
If you like to travel, there could be something very interesting to collect: rocks and crystals from all over the world with different colors and shapes; sand with various colors from different beaches in the world; fridge magnets or postcards for special memories; small arts and crafts or trading cards from where you have traveled to. Just use your imagination to think of something to collect for lots of fun.
Here is an article about some unique collections, which could inspire you in some ways.