easiest are obviously gonna be platties and guppies ....
corydoras for the bottom would be pretty sweet as well, maybe some loaches to spice things up, i can definately advice to get a dwarf loach, the things look beautiful and are very active and crazy :)
you could also try some neons or tetras, but advice well in the petshop, maybe bring a book cause some tertas/neons can be quite dificult to handle....
i use a rule of thumb for how many fishes you could keep, i count 1L water for 1 cm of fish, so if you have a 60L tank you can keep 60cm of fish (ie. 6x10 cm loach, or 20x 3 cm guppy, or 12 x5cm corydoras or 2x5cm dwarfloach + 2x5cm corydoras + 10x 4cm platties)
if you don't have any waterplants you can also get an apple snail, these things look super cool and they eat fishpoop so your tank remains clean for longer time :)
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