I wouldn't try the hamster ball. For one thing the frog's probably not heavy enough to move it well, and for another I can just see it ending disastrously.
I understand the impulse to allow the frogs some freedom for exercise though.

I'll admit to letting the more adventurous one of my frogs out on hot summer nights, letting him climb the walls. But I do try to frogproof the room by putting away things small enough to eat and vacuuming so that there's no hair or lint on the floor.

You'll have to weigh this against how upset frogs get at being handled. If the frog's nervous enough to pee on you I wouldn't do it. Frogs aren't quite like humans and are patient, perfectly content to sit and wait for bugs. And of course there's always the chance of being lost or injured outside the tank. But I do feel like they should get the chance to get out at least sometimes, so right or wrong, I try to compromise.