For Christmas Milton got a book and papers for making paper air planes. Today we have made dart planes, gliders and stunt planes. After hours of flying them and asking for my help to fold them he decided that he wanted to learn how to fold paper boats. Here is a site with very easy step by step, with pictures, lesson on how to fold your own paper boats.
You could take them to the bath tub or revisit the lesson when the warmer rains return and head out to a gutter or stream for some outdoor fun.
UPDATE: Paper boat did not work so well in the tub. Boys ran back down stairs within a minute reporting sunk and torn boats. So then we tried aluminum after watching this motivating nautical video.