At this point, Luffy has the best chance of meeting Bon Clay again, and I really don't see Sanji learning any moves from anyone on that island. Okama Kenpo isn't his style. As everyone said, leg strength from running might be something. Maybe he'll kick one of them to find out that they are all super sturdy thus making his body adjust to kicking such hard things. In this way he'll not only get stronger with his muscles, but his bones will become much stronger as well.
Luffy's is obvious. His haki has developed a lot from knocking Mumumbo(?) the love bison out, to freezing the Boa sisters in their tracks(haki users) and knocking countless people in the crowd unconscious and foaming at the mouth. This will continue to develop.
Usopp will get new gear. After all, he's on an island full of man eating plants, weird plants and giant bugs with Heracles. Maybe add some beetle horns as projectiles or a make shift close quarters weapon or shield. There may be some plants with different properties that he could use with his ammo. He's great at inventions so he's got plenty of material to work with.
Nami's is obvious as well. Her weather attacks will become stronger and maybe get more knowledge on these things.
Chopper….hmm...I don't know, but it has something to do with the giant birds....eventually.
Zoro needs lots of rest now, but I don't know how Perona will fit into this.
Franky will study all of the wondrous things they have at the most futuristic place on the planet and make some upgrades. I'm anxious to see what he'll do with it.
Brook? Eh. Your guess is as good as mine.
Robin will learn a lot here with so many centuries of history right there for the taking. She's not is seastone cuffs, so she's relatively safe. Other than that, hmm.