I'd definitely point you towards a Pacman frog. They're really easy to care for, they get to a pretty decent size, and they eat like hogs. The thing is, they don't really MOVE. If you want an even more impressive frog that is more "reactive" I'd recommend an African bullfrog. The thing with ABF's is that they need more in terms of space, and I feel like it might be a bit of a challenge giving a growing/adult male enough to eat.