I'm not sure where your skillets and specializations lie so I'll just rattle off some stuff from a solarpunk standpoint
A big piece of solarpunk is in reusing what's already made, rather than in making new stuff. Maybe an app or ROM that'll make old cellphones useful for some single-purpose task? Like as a display for linked sensors in agriculture or something? Bonus points for focussing on phones that aren't supported by lineageOS etc. Full disclosure, I don't do anything with mobile development so I have no idea how much work I'm putting in front of you with this one.
Edit: this actually wouldn't be a hardware task
Another aspect is the way it connects nature and technology. I'm not sure if there's a gap in prosthetics for animals that you could help with?
When I think about solarpunk hardware, I think I'd want any new technology to be simple, robust, and fixable, even if that makes it ugly or bulkier than it could be. The right to repair should be baked in as much as possible. I think where possible, parts like cases should be sustainable (recycled materials) or biodegradable. I have no idea how to emphasize all that in a hackathon but maybe you will?
I'll add more as I think of them.