Answer:
Wait for the next island, or sharpen your skills against other players in the multi-player games on Main Street. You also have these games in your item bag. If you have completed every island on Poptropica, you can always make a new character and do them all over again.
You can reset any island and play it again from the start, although you cannot earn any more credits for winning with the same avatar. You can also play Reality TV island as many time as you like, and each win will raise the total shown on your Island Medallion.
You can just play around in the arcade rooms to get your battle ranking up. You can also spend your credits that you earned from beating islands on Poptropica.
You can buy the haunted House mini-quest or Dr. Hare's Secret Lab from the store, or play the Earth Day mini-game for free.
UPDATE
Be sure to watch for advertising mini-games in 2011 and 2012. They will only be on the site for a week or two. (You may have to have a certain age or gender of avatar to play them.)