Aside from work, I've recently been working on a Nerf gun project. I've started writing in my blog about the developments. I'll detail how I design the gun including a bunch of math most people would find useful. A few folks here should find this to be interesting.
http://trettel.org/blog/cat/fang4
Once this Nerf gun is done I think I'll try a new water gun and go through the same process.