Whatever you do, don't use bleach until the urine is all out, it will gas you. Try detergent and borax, let it soak and then scrub with a brush. Then you can bleach it to sterilize it. What a nice thing to do!
When we had rabbits what I would do (although it wasn't that bad) was take the cage into the yard and use a lot of soap, and high pressure water from the hose. After that I would let it dry out in the sun, then I would clean it with a household cleaner like fantastic or something like that. Of course it has to be totally dry before any animals can go back in it, but since you are donating it, that shouldn't be a problem. Congratulations by the way that's a nice gesture.