Animals - American coins are full of Eagles and Buffalos. Other countries feature different animals. One could try to assemble a zoo.

World - Coins from around the world are often beautiful and many times less expensive than American coin.

Errors - The US Mint is not perfect. Sometimes errors get released. Many people are fascinated by mint errors.

Varieties - Some folks become so specialized in their series they learn about the minute differences between the various different dies used to make coins.