I got thinking about doing something in the robotics area, and came to the realization that there is one very basic concept - move an arm around a fixed point.   Add another arm to the end of the first, and maybe another to the end of that, and you can come up with some pretty versatile machines.  

This is a work in progress, to be added to as ideas appear.   Some features already planned - a pen at the end of one or both arms to plot a trace of the motions, sensing multiple keys pressed at the same time (for two-hand control), and maybe a third arm.

Somewhere along the way, I noticed how very similar this program is to CircleArt (working on getting that one working on the site).   But CircleArt plots whole circles at each position, where this one should draw continuous lines, more like a Slinky.   In fact, since we've opened the brand-name door, it would be just like the old favorite, Spirograph.

A third (and again similar) program suggests itself - something I've always wanted to build out of wood - a working orrery, or planetary model.   I want to model the Sun, Moon, and Earth, to demonstrate day and night, moon phases, etc.