IT really depends on your budget and what you think you can handle. You could try a plant tank, with livebearers, a brackish tank with a few knight gobies and mollies, or even a small reef tank with a few fish. A 29 gallon is a nice size tank but you can only go with so much. Probably you can make it an exciting tank with the many possiblities. Like I said, it all boils down to budget and how much work you want to do on a daily, weekly and monthly routine.
jim