Ummm, might sound a little ordinary, but what about an American toad? They're free, they're cute, and they aren't tropical so you don't have to worry as much about temps and humidity. As soon as my brother takes my catfish I'm going to empty the ten I'm cycling now and get a bunch of toad eggs. Once they get their legs and stuff I'll let them all go outside and keep one for a pet. And you can get the males to sing on command, because when they're mating I often sit near the water where they are and do this little thing with my tongue, and they get all huffy and sing back lol. Males are smaller than females (like by half or maybe a third), but I think you could keep one in a ten gallon no problem. And they eat anything that freaking moves, so no picky eaters to deal with. Just my thought...
Or, you could go with a smaller snake, I think corn snakes don't get all that big, but I don't know for sure, the only snake I ever actually had was a baby black rat, and they get I think about 4 to 6 feet long.... The only problem is if you're squeamish about live food...