We have a male betta fish and we're very happy with him. He perks up when I turn his light on because he knows he's being fed. He's quiet, doesn't eat much, doesn't require much space or long walks, is hypoallergenic---and he's fun to watch. Dentists sometimes have them in their offices because watching fish is soothing to patients. Male bettas are usually colorful with wavy fins, though we have a pale almost white halfmoon betta (pricier than some of the others but we love him).
Male bettas are aggressive toward other fish so they are always in a tank alone. You can buy a one-gallon tank with or without a filtering system and you clean the tank about every two weeks. You use regular tap water and put in a fish additive for the tank, but let it get to room temperature before using it. If you go away on vacation, you can buy 7-day solid food shapes that dissolve in the water during the week, so no worries about fish-sitters. I feed the fish most days because then I know it's done and we don't overfeed him by accident.
Bettas live up to 4 years. I've read articles where doctors suggest getting a fish or another animal with a short lifespan to teach kids about death.
The pet salesperson recommended betta fish over goldfish because they're cleaner. I don't know. I had goldfish and guppies as a kid, but I wasn't the one cleaning the tank!