There are so many inexpensive options around for powering gadgets by solar energy.
Here is one of our small solar LED keychains that cost less than $1.
We use them so much, they inevitably wear out or bust. They have plenty of usable parts even when broken, though.
Here you can see I've removed the LEDs and bad battery to add a new one and wires tapping into the voltage used to power the LEDs. In sunlight or off the battery the circuit provides several hundred milliAmps at 3.6V. Plenty for whatever we want to power.
Here are various power sources we've used.
Walmart sells the round ones for $2.50 now with 'AA' size batteries useful for recharging USB devices.
The big square one has several batteries and powers Big Foot's eyes...
Of course they work great as advertised, to light up the area and decorate the drive.
Here you can see the small 'spy' video camera keyfob. An $8 video camera that can record more than 4 hours of video can be quite handy.
Here we are using the solar pack to power the video camera. It is nice the solar cell has an on/off switch.
The $2 microcontroller can turn them all on, start recording video, snap a photo, and even run a couple servo motors to pan/tilt the camera. Small and cheap - just waiting to be put out in the field.
So many options are begging for you to hack and reapply. What ideas to you have?