Mmmmm, I may freecycle it as a piggy cage.
Rather than a littertray, you could use it as an interesting 'dig box'. You could do sand one week, soil the next, compost the next, shreddd paper the next, etc. It's great environmental enrichment.
I have a smallish indoor cage too, the babies have it in the livingroom with their litter tray in it and the loads of hay, it truely is great because it stops the floor getting messy! I dont shut them in there though. I also have a water bottle attached to it to, as well as a water bowl on the floor but not in the cage if you know what I mean!!! If you buy some NIC cubes, you can make an enclosure with the base of the indoor cage in it