Omg so many!!! You can make it an Anenome tank with out any corals, or you could do corals and all sorts of invertes and fish.
With a 20 g you could probs have at most 4-5 fish, if some were small. There are a multitude of fish you could get, gobbies, blennies, damsels, pipefish, seahorses, could do a stone fish w no coral, firefish/dartfish, there are tons!
Invertebrates- harlequin shrimp are soo cool bit they eat live starfish so that makes for a messy tank w more frequent water changes. There are fire shrimp, cleaner shrimp, sexy shrimp, Anenome shrimp, peppermint shrimp, pom pom crabs, emerald crabS, porcelain crabs, tons of diff colored hermits( including electric blue And Halloween hermits) you can also get a sea cucumber or sea hare though they are harder to keep. You could get arrow crabs or a pistol shrimp, which can pair with a goby to make a fun duo. I also have a coco worm and a Feather duster. And don't worry many of these inverts can live together.
Corals- lots and lots to choose from, I advice you do a bunch of research before you buy anythig so you don't waste your money, time and your tank gets a good first start. Easiest to start with are soft corals or large polyp Stoney corals.
Hope that helps, you can create some really cool tanks with a 20 g tank, do some research and youll find plenty of examples.