what do you like? Which sounds more appealing to you?
1) a bunch of small, colorful fish that are pretty peaceful, swim in schools, and are relatively inexpensive to maintain?
2) a couple of small- medium sized fish, sometimes colorful, with fascinating breeding behavior, aggression, and lots of personality?
also, do you want plants? Some fish eat plants, other's need them to hide behind and to maintain water quality, others will simply find it entertaining to uproot them and allow them to float around the tank. You really can't have it both ways.
If option one appeals to you, I would go with a couple of schools (6 or more fish) of tetras, barbs, or rainbowfish (not all rainbows will work in your tank). Add in some hardy plants and you'd be good to go.
If option 2 appeals to you then I would look at doing a species tank of cichlids. You could do smaller fish like german blue ram, bolivian rams, any of the apistogrammas, or kribensis (pelvichromis pulcher). The are amazing looking fish, fit nicely in your tank, are excellent parents with interesting breeding habits, and you could probably sell the fry (baby fish) to local stores or hobbyists. If you want something a little bigger and meaner you could do convict fish, flyer cichlids, blue eyes cichlids, neets (neetropulus), rainbow cichlids, or firemouths. You might be able to do angels (pterophyllum scalare) but they usually like a taller tank. Convicts are super easy to breed. They say that breeding convicts is as easy as "just add convicts and water," but the fry are worthless and nobody will want them much less pay you for them. Some of the smaller pike cichlids might be cool. You could also do jewel cichlids (Hemichromis bimaculatus) they're a medium sized gorgeous cichlid from african, but they do not require the more alkaline water requirements of most african cichlids. The thing about cichlids is that they hate plants. No plants in the cichlid tank, just fake ones and then be prepared to have them dug up. Rocks, driftwood, and clay pots are good decorations for cichlid tanks, and a fine substrate.
Speaking of african cichlids, there are a bunch of shell dwelling african cichlids that you could put in there, but I don't know that much about them. I do know that african cichlids like hard alkaline water, and that the rift lake cichlids figure prominently into the argument for evolutionary biology. Apparently scientists believe that all the rift lake cichlids were initially one species of fish that got trapped in the rift lakes when the ocean receded. Could be an interesting conversation piece.
I hope this helps. I just got into the hobby myself and I scoured the internet incessantly looking for things I could do with my weird tank (37 gallons). Here's a couple of sites that helped me out.
This one is pretty much copied verbatim from a book about setting up freshwater tanks by david e boruchowitz : Stocking Schemes - Freshwater Aquarium Setup - Freshwater & Saltwater Aquarium Forum
this one is for cichlids. look under where it says cookie cutter tanks:
Aquarium Quick Reference Resources - Cichlid Keeping
one other site with good information actually sells fish!
basically you don't want a fish that gets much bigger than six inches. If it's a standard 30g then your tank is only 12" wide. Anything much bigger than 6" is going to be a bit cramped when fully grown. Cichlids that pair off to breed will kill anything that messes with their fry. Also even if they don't breed, they will probably fight each other a good bit and will pick on/kill the weakest fish...