I'd go for a 20-gallon aquarium. Big enough to have variety, but not so big that it would be overwhelming. There are plenty of kits out there that would be ideal.
Also, if you want minimal maintenance, stay away from tetras, livebearers (like guppies and platy) and goldfish. Tetras can be extremely sensitive to water temperate and water quality, livebearers can reproduce so much that your tank will crash from the bioload, and goldfish are very messy.
I'd go with a large school of white cloud minnows or zebra danios. They are some of my favorite types of fish. While some people sneer at them for being less colorful than some of the other fishes, I personally think they're quite lovely. They're great little fish, and very active. They look absolutely amazing when they're zooming around in a school together, just glittering in the light.