Jawfish require really deep sand beds of a minimum of 6". 8 to 9" is preferable. With a 20g tank, you would almost have more sand than water, so I don't recommend a jawfish.
There are a lot of options out there!
Clownfish are neat, and your girlfriend can watch Nemo and Marlin all day. lol!
Firefish Gobies are entertaining to watch, and they are pretty colorful.
Purple firefish gobies are interesting too - same thing, only more rare because they are purple!
Yellow watchmen Gobies are neat to watch.
Neon blue stripe or gold stripe gobies are fun to watch and will clean your other fish and stay small.
A small pygmy angel would do well in a 20g tank if the rest of the tank is a low bioload.
A shrimp and goby pair would work pretty well. The shrimp and goby come out of a hole and the shrimp is blind so the fish protects it and lets it know when a predator is around. Very neat little symbiotic relationship there.
Yellow and green clown gobies are neat, and they perch everywhere like a yellow watchman goby.
green banded gobies are neat too
Umm,... ... um.... ummmmmmm... ummm...
A bicolor blenny would be nice. Just one though.
For something that is mildly aggressive you could go with an orchid dottyback or a royal gramma. I think those are really cool fish.
Of course, you could go ultra cheap and get some blue devil damsels and watch em' stake out their territory. lol.
The list goes on and on...