Solar charge controller / power supply. Basically a circuit that will take practically any voltage from a solar panel or other sporadic source, and generate a float voltage for batteries, and also have a power output for actual loads and maybe even a 120v inverter. It would then monitor and log stuff like solar output, battery input/output, and shut down load if battery is too low. Could add other bells and whistles too. This could be contained in a box that has the battery in it too and cig lighter plugs, storage for misc components like flash lights and radio. Basically it would be a solar emergency power box.
I'll actually be building something like this shortly as I'll be going on a camping trip and thought it would be neat to have some portable power for cell phone, lights, and stuff. Sure the camp may provide power but more fun if you produce it yourself and have to conserve and control your usage. Though I'll be using off the shelf components just because it's faster and they were on sale.
But if I end up really liking this I might build a better one that is better thought out. Most charge controllers also require the panel to be a high enough voltage, but I imagine if it could use a voltage doubler it could take say, 10 volts and make 13.5 with it after going through a smps circuit. I'm not sure if there's even any charge controllers that do this and perhaps there's a reason why, too.
I'm a beginner myself so can't help much with technicalities, just an idea.
Another fun idea is an induction furnace. Basically something that will melt steel using induction power. Kinda like an overclocked version of an induction cooktop.