Stickers, banners, streamers, ribbons. You may want to check with Sea World ahead of time to make sure they'll allow balloons or any plastic type streamers, though. I think a lot of zoos and places like that don't allow ballons and other similar items (I know there's no straws at our local zoo), because they can be a hazard to the animals. I'd hate for you to get there with the wagon all decorated only to be turned away at the gate or have to take away all his cool balloons.
I'm planning on taking my son to SeaWorld for his 2nd birthday and want to decorate his little red wagon so everyone knows it is his birthday. Besides balloons, what other ways can I decorate his wagon?