Platies or zebra/leopard danios. Either is exceptionaly hardy. Danios should be in groups of 5 minnimum in at least a 20 gallon. They get to just over 1". Platies are livebearers so watch that you are either keeping 2 females to every male or all one sex. They also only get to about 1.5", with females being a little larger than males.
White cloud mountain minnows are also hardy but they are only borderline tropical so acoid them unless you are planning a tank with slightly lower temps. They are schooling fish and, diealy, should be in a group of at least 5-6. They get to just over 1" with females being considerably fatter than males.
Not all livebearers are hardy fish BTW. Even the popular fancy guppies are no longer particularly hardy. Mollies, also, though hardy when compared to some species, are not all that great starter fish. They also get to 4" so are quite big. On the other hand, the mosquito fish (heterandria formosa), is a very hardy little fish but, because of its size (under 1"), it is often in danger of being eaten. It tolerates lower temps very well.
Bettas are quite hardy but must not be kept with gouramies or nippy fish and don't go well with guppies or anything that looks similar to them. If you get females, they must be in a gorup of at least 4 to minimize aggression. Males should be kept alone and much care should be taken in selecting his tankmates.
Another hardy species is the bronze and the peppered cory. I have found them to be incredibly adaptable. The only downside to them is that, because they don't have scales and are bottom-dwellers, toxins in the water can have a more serious effect on them than they do on the others mentioned. However, if the tank is already cycled, they are just as good a fish to start with as any.
The american-flag fish (a killi) is also very hardy and, like the danios and minnows, they can tolerate lower temperatures quite well as well. Be careful not to over-crowd them though. They get to 3". In character, they are much like gouramies but often seem to behave more like cichlids. Still, they are peaceful with other fish and will also munch algae.
Three-spot gouramies are also very hardy but you need to take care to know what you'r dealing with as, the males especialy, are quite aggressive, Realy, if you get a three-spot (which get to 6") you should stick to the one and don't get any other gouramies/bettas/related fish. Their fine with most other tankmates though
. These are beautiful fish and come in several color morphs - amongst these, gold, opaline, blue, cosby, platinum and lavender. The opaline color must be my favourite.
With all these fish, you could actualy set up a very nicely balanced, fully-stocked tank.