My favorite shapes to play on are 2-person PVC pool boats and big 6-foot Shamu whales. I'll also happily play on a big at swim tube, those work too
I'm also very partial to whatever material Intex uses. I have no idea what recipe they use for their toys, but it feels wonderful. Sevylor is a good second place, but their seams are very weak...at least for the type of play that I enjoy.