Get guppies. They are very entertaining. They are little fish that give you more little fish, which also give you more little fish, which give you more little fish after that, which... You get my point. They're rather fun, though. I have a cobra guppy pair that I raised from fry. They're very beautiful, and fairly hardy as well. BUT, if you want a fish that absolutely WILL NOT die even after four years and you're starting to think about killing it yourself before it bites the dust (that's what happened with me) get a zebra danio. Man, I hate those things. They just won't DIE!!! That might sound mean, but you just get so SICK of them after a while! Anyway... Guppies are nice, but they can die. Zebra danios aren't nice, but its next to impossible to kill them.
Your choice. But I'd go with the guppies. But you might have to keep on buying them. I did. 'Course, I used to have a talent for killing fish. Not anymore, though, thank goodness!!!