If you want no brainer easy then any of the live barers , so guppies, platties, mollies, swordtails, etc. And platties are probably the easiest to keep out of those simply because many guppies are interbred nowadays and can be fragile.
Something next level like bristlenose plecs, corys, leopard or zebra danios, all still nice and small, and rosy barbs, cherry barbs or similar.
also a little larger, 3 spot gourami or lace gourami, or even Siamese fighting fish, but growing on the young is a challenge there as you need to separate males at an early age.
convicts and jewel cichlid are easy but getting rid of the young can be hard so stick to something like kribensis.
A twenty would be fine for a pair of kribs.