Both my sons have always enjoyed making Paper Airplanes. The price is right and the mess is kept to a minimum.
This site is great! 15 Free Paper Airplanes to print from your computer. I've recycled paper and used the backside I've already printed to print these out, my boys didn't care.
If you don't have a printer available, check out Alex's Paper Airplanes. There are several different planes here with folding instructions.
Here's one of their favorites. The Best Paper Airplane. The instructions are easy to understand and the pics are great.
Newspapers make great paper airplanes as well as pages from old magazines.
Here's a site that has planes as well as boats! Origami Kids
What frugal activities do you do with your children to beat summer time boredom?