Watching this one with interest ... considering moving our gps from basement to sitting room because they seem so bored in their cage today ...at least that way they can interact with us a bit more ....
I do let them run around on the floor a bit (have bought one of those fleece tunnel things that they like to hide in) .. and they generally 'lift their heads and/or squeak' if they want to pee or poo, so that's the signal to return them to their cage ...
I also hang up a few treats in their cages (hard foods/fresh foods such as broccoli stalks) and make sure they have fresh hay morning and evenign. I give them quite a bit and they run through it and over it and under it which is quite sweet to watch ..
They quite like playing around in half a hollowed out melon skin as well.
Other than that, I'm stumped .... although I have heard of people sticking cardboard boxes together (then cutting holes in the 'walls') to give them indoor 'rat' runs, apparently they quite like that . Not sure how cardboard stands up to copious weeing though.
BTW Rusty Bear, I'm no expert, but I didn't think it was advised to keep rabbits and gps together any more ....somethign to do with bullying apparently ... yours sound as if they are firm friends though...!!