So many choices...HELP ME PICK A PET! :D?

My mom got a puppy for her birthday, and long story short, she would like to get me a small pet. The rules are: Nothing large like a cat, dog or parrot. No reptiles; I like my room a little chilly. I have a very large tank that was originally set up for a koi. So my choices are: Koi fish, guinea pig, ferret, hedgehog, parakeet, or any other small animals. I'm not asking which one is easiest to care for since I'm responsible, but I just want some honest opinions :D thankies <3