Heat rocks are really outdated, and there are no animals I suggest using them for. Their reputation for malfunction and burning inhabitants is all too common. If you're looking to do plants, frogs are a good place to start. There are a ton of options, and you'll have to look around and see what you like. Avoid heavy burrowers, like pacman, pixie, and chubby frogs. You can also try smaller lizards, like an anole, or long-tailed grass lizard. You could try a day gecko, too, though I'd recommend some reptile-keeping experience first. Now that summer's coming around, you could pick up a rough green snake. It's a small, very attractive US native. And, they're insect eaters, so no worries about having to feed rodents.