Coins often have themes such as war memorials, cartoon characters, animals, space, flowers, and more.  Sometime coins are sold in subscriptions, whereby you subscribe to the series of coins for that theme.  The coins are then shipped to you as each coin in the series is released. These series can cover several years, in that you would complete your collection of the series after these years have past. 

Here are some examples of theme coins (animals, movies, holidays, sports, and much more): click the coin to go to the theme page