Sadly 30 gallons is very small for a saltwater aquarium. It wont allow a lot of room for larger marine fish or a great deal of marine fish. What I typically recommend for people w/ tanks of this size is to keep them invert heavy. Since shrimp, crabs, anemones, corals - etc do not have near the metabolic rates - a.ka. they dont foul up the water as bad; as fish then having plenty of them in a 30 gallon is an option.
I noticed you placed your question in the FOWLR
section of the site, although I would reconsider that. Some people like inverts as much or more then fish anyhow. My recommendation would be this
Add some T-5 light of at least say 90 watts. Get some inexpensive Condy or Hawaii'n Pink Tipped Anemones. You could keep some zoo polyps, soft corals, or star polyps if you want to keep it simple. Then look into Skunk Cleaner Shrimp, Coral Banded Shrimp, Fire Shrimp, various starfish, hermit crabs, etc - etc. As for the fish you can keep I would recommend the following animals:
False Percula or Percula Clownfish
Any of the clown gobies
Six Line Wrasse
Neon Cleaner Goby
Naturally you couldnt house all those fish but rather choose a few from that group to try.
As for inverts I would recommend:
Skunk Cleaner Shrimp (2)
Coral Banded Shrimp (1)
Fire Shrimp (1)
Possibly sand sifting star or brittle star
Remember you will need good quality water with low nitrate values and a decent protien skimmer, possibly a CPR BakPak or Aqua C Remora Nano to make a small reef set up like this work.