50 gallon tank... have you considered a small turtle? Believe it or not they are pretty interactive with humans. Not like rats but I'm thinking get a Martin's cage for rats, fill that tank with something else that's more active than a snake.
Hermit crabs are fun to watch, they're active at night & a 50g tank would hold 10-15 or so. There'd be a lot of action with that many.
What else could do in a tank? Bearded dragons are interactive with humans... what else?
Gotta think more about it. lol
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PHRatz