I find that if I give my nephews tape and almost any other material, they can come up with some pretty cool stuff all on their own. My nephew happened across a bag of popsicle sticks and a roll of masking tape and soon whipped up this delightful -- and entirely seaworthy (or at least sink- and bathtub-worthy) little raft.
Here he's using it with his battery-powered toy hamsters, but it also works really well to float plastic or foam toys in the bathtub. This is a fairly conventional raft design, but there are, of course, innumerable variations of other vehicles and structures as well.
Popsicle sticks are the perfect size for small hands to manipulate, and great for fine motor skills (lining up the sticks side-by-side) and counting practice ("how many sticks should we use for our raft?") Who knew you could find such a remarkable toy for just a penny apiece! What creations have your little ones made with popsicle sticks?