The items on my list are half concrete tasks and half topics to learn. The subjects are very diverse, I have a broad range of interests.
Here's a few from the top:
1. Learn enough classical mechanics to do a stable n body simulation
2. Build a piece of furniture with mortise and tenon joinery
3. Implement a prototype compiler for that GPGPU/shader programming language I've been dreaming about for years
4. Get an Indian fighter kite and learn to fly it
5. Implement a multiple revolution Lambert problem solver to plan the badassest Kerbal Space Program mission ever
6. Write a simulator for gyroscopic motion and compare the results with my toy gyroscopes.
That's enough to occupy my Saturdays for the next five years. Both, the sunny and the rainy ones.