what species for a 20G long?

October is within a stone's throw away, and I'm getting excited for hitting up NARBC. If everything goes to plan I will have an empty 20G long, which OBVIOUSLY means that I have room for another roommate.

What kind of roommate though? This is where I'd love you guys to help! I want to compose a list of species of all sorts, as right now I have no clear idea of what exactly I'm looking for. Another snake? A lizard? An amphib?

My emphasis might be on something a little less common than what is found in your average pet shop.

Thanks!