Best fish or other to place in small tank?

i’m moving into a static caravan soon as my permanent residence and i would like to have my first aquarium, something small, low weight and doesn’t take up much space.

my parents have kept fish all my life but always tropical fish in a large tank, or a pond with goldfish and similar. they aren’t well versed in different kinds of tanks and fish.

i assume my best bet will be some kind of shrimp tank, but i wanted to see if anyone here has any other ideas!

thank you :)