Already a fun and basic activity that allows for creativity. So you're off to a good start.
With out much more materials you could make an "advanced" boat challenge using cardboard and tape. Keep the size smallish still, but give the students set cargo (like action figures, toys, or other odd shaped items) they need to support. This may make the shape of the boat as important as the volume.
And if you've got some $ to spend, buy some small motors and battery packs and have the kids build propellors for the boats!
https://youtu.be/78fvm0MFaOs