What are some cool tetra?

I have a 20 gal tank with some goldfish that I'm gonna sell or give away when they get bigger and I was thinking of tetra when they are gone. I know shoals of 6+ and I wanted to know what ones have interesting behaviour. I don't really care about looks or colors. Do you have any advice and how many can I keep if i want tank mates and what tank mates would work. Yes I will get a heater when the goldfish are out. Thanks for your advice.