Funny post - gotta love Reddit. I'd think a rat snake would be a great addition. I don't think I'd ever recommend a solitary garter snake (sorry if anyone reading has just one, no offense intended). They just do so much better in small groups, and they're a delight to watch - the person asking that question could definitely keep a group in an enclosure that size, with climbing and swimming options. My group of four female Oregon Red-spotted Garters is endlessly entertaining. I'm just waiting for their 4x2x2 enclosure to arrive.