Simple and next to free bath toys to make e.g. in a hotel room:
All you need is sponges, straws, scissors and glue. Cut the boats and the sails into shapes, glue together, stick the mast through and set the sail. Done.
On the picture you can see what me and my kids did, I have seen other variations of sponge boats too. I got the idea first from a German magazine for parents called “Die Eltern” (which I love for their easy, free and fun little ideas for crafts).