Well first I'd say to definitely put a folded fleece blanket or something similar up there. Back when I got my first bunny he tried to jump up on his once and he got his little foot twisted down between the metal. Luckily he didn't break his foot :foreheadsmack:
You could try either training them to not get up there, i.e. with a clicker or a small spray bottle. Or you could put something up on top. Folded blankets or maybe a large bag of bedding if your cage is one of the standard sizes? Anything would work really, so long as it's not anything heavy or a bag of food that would tempt them to get up there more.
Also you could try having other things in their room/area for them to jump up on. They might just want to get a vantage point I have 5 storage bins (filled mostly with yarn and some old clothes..) that I keep in my rabbits' room and they adore getting up on those.
If it's not an issue for them to be on top of the cage, I would simply put something next to it for them to use as a step to help them get down after they're on there, so that they don't get stuck
My guy loves to get on top of his NIC condo. It's large enough for him to flop and survey his room. I'm with everyone else that's responded. Put something soft on top so bun doesn't twist a leg, put something next to it that bun can use as a step up and down (non-slip), and let those babies enjoy! :bunnyheart