I'd go with a betta. I have one named Sapphire that lives happily in my office in a 3 gallon tank. I use plastics plants and a ship ornament for him to hide in, although he much prefers the plants. I didn't care much for them at first, but in my office, I have restrictions too. Can't understand why they have a problem with a 75 gallon tank in my closet of an office... Anyway, bettas do have a personality and would probably a good idea for a 5 gallon. Just make sure your filter isn't too strong. I lost the first betta to a internal filter that was too strong, he got sucked up under it and died.
