It is pretty difficult to come up with stuff that is smaller in scale.

Maybe a solar powered bracelet that scans for various signals? In this day and age, people should probably be more aware of remote tech that can be used against them like air tags being slipped into vehicles or spy cams in air bnbs. Having a device constantly check for wireless signals and then encrypting that data and sending it to a phone or something could be useful. Especially if you do not need to worry about charging it thanks to the solar panels (assuming it gets enough sun), might want to include a charge port just in case though.

Not so much solarpunk as much as a neat little gadget with a solar panel though. It would probably need an arm cpu and a custom pcb too.

I wish you well with your project.