I second the planted betta. looks really nice with sand and plants a small drift wood. or maybe a few endlers. its funny cause i have monster tanks and im actually dying to do a small tank like that for my kitchen.
I have a striped caribbean hermit in a 5gal with some live rock. he's doing pretty well and is big enough to be easily seen all the time. definitely a good idea.
Sw hermit crabs? problem with small sw tanks. they really arent for fish the parameters, salt levels and all that are too stabalize. although if u can do it and commit to it i would greatly recommend these cute little sw gobies called panda gobies. they are tiny. i had a 5 super nano sw a while back. with these and hermits.
I'm glad Channarox doesnt read my posts anymore because I'd never live it down for suggesting it years later but... Mosquitofish are really awesome really cool personality