You don't do anything. Just wait for the next island to show up and play on it.
wait for the next island to come out
I have personally have gotten all the medals on poptropica too. My advice to you is to make a new person and see if you can beat the islands again. You could also try chatting and playing with other...
hang around
nothing
U can do anything you want like battling other players or chatingor you can wait until another medal comes out ;)