My son made a paper gun that actually shoots. It's pretty cool for just paper and glue, but it took quite a bit of adjusting to get it to work properly.
Tinkering is one of the main ways my son learns and therefore, there have been several posts on Highhill Education that involve tinkering and learning.
Flying a Kite
More of these types of posts can be found on my Science Page. Especially towards the bottom when the activities become more engineering oriented and free form.
Check out these great blogs full of educational activity ideas.