clownfish (ocellaris or percs) are always a good add. Don't opt for other species that get bigger and/or more aggressive.
yellow watchman gobies are great...a lid or screen is a must have to prevent jumpers
yellow-tailed damsels are the only safe damsels I've found. I've kept many tanks (nano or otherwise) in the last 13 years and the only fish I've seen yellow-tailed damsels fight is another yellow-tailed damsel. Don't try 3 in a 120g...they will fight. 1 in any size tank is a good add.
blue or green chromis are like the damsels. Get one and enjoy it. They're active, pretty, cheap and don't cause problems.
Cherub Angel - I had a good experience in a 30 cube with one. if you get it, skip the yellow-tailed damsel. They look a lot alike. I'm not sure if they'd beat up on eachother, but it's not worth the risk. Also, in a nano, you don't get to add a lot of fish. Even if they don't fight, why have two fish with the same color pattern?
Coris or Flasher Wrasse. Tons of activity with these guys. Snails are safe IME, shrimp are at risk. I'm not sure about hermits, but I don't keep them since they kill snails
There are a few others that I'm blanking on, but those are pretty safe choices. Stick with a pair of clowns, a goby and 2 other fish and you'll be safe for a 30'ish gal tank. More than that and it'll be a little crowded and look more like a fish tank than a reef tank.