Can't decide what to put in 30 gallon!

African butterfly fish?

Archer Fish! You'll have to look for some freshwater acclimatized, or go brackish in your tank but if you leave some airspace at the top of the tank and put some vine work up, feeding time will be a show every time. Been thinking about putting together a tall tank specifically for a couple Archers.

It goes for $14 from live aquaria - not really expensive by my standards and IME local prices are cheaper.

I don't think they're very common - actually I haven't even seen one in person (but I'd like to point out that I don't visit the LFS more often than once a month, if that). However, it shouldn't be very hard to have your LFS special order it for you if they don't regularly stock the fish.

Also, I have to comment on the archer - not a good idea IMO. Keeping brackish fish in freshwater is ultimately going to be what kills the fish, provided that something else doesn't kill it before hand.