Not going to lie that sounds like a really hard context... like you've said a bin is the obvious choice but if you can't involve systems and control then making it fun and interactive could be hard.
Can you use any electronics at all?
Some sort of sorting game might work maybe? With kids having to sort out recyclable materials from those which are not.
I'll keep thinking for you but it does seem to be a really hard brief. I can think of incentives to encourage kids to recycle but they're not really something which could be made as as physical product. The Zoo and recycling just seems to be an odd combination. Let me know how you get on though, following with interest!