At the moment, when you visit a friends island, the only thing you can really do to help them is buff their buildings..
I'm thinking if you visit a friends island and find a collectible, it should be possible to click it and send it to the inventory of the island owner, or it would appear in their inbox next time they login similar to how the adventure island cauldrons work.
This might actually encourage people to explore and interact with each others islands more, adding another bit of fun into the game.
What do you think ?