Coldwater fish are easy. Goldfish are an ever-popular choice; I myself dislike them, but it's whatever "floats your boat."
Minnows are pretty good, but if you buy them from a bait shop, they're likely to be poor in health, though when put in a properly balanced tank, they'll recover from abuse very quickly. Most of them are actually very hardy fish.
If you really want, you can catch your fish locally - But never, ever, EVER mix them with storebought fish. You could introduce a disease to the tank that your river fish may be immune to, but the fish bred in captivity won't be able to cope with it. I know someone who had a 180-gallon tank and kept bluegills and oscars - you talk about beautiful! They'll eat pretty much anything you throw at them, and they're easy to care for. Be warned, though: In captivity, wild fish grow large, and they grow fast.