If you don't mind spending a little bit of money on a custom sump, you could get something that will be able to fit inside. They do make a 16 and 20 gallon tank that has the same footprint as the 10 gallon, but that won't do you any good, you need something longer.
I personally like sumps, you can hide most of your equipment in there and they allow the tank to keep a constant water level.
I would suggest looking at leather corals, soft corals and mushrooms. Most of them tend to be fairly hardy and should do well for you (with proper lighting and water quality).
"Simply put, you believe that things or people make you unhappy, but this is not accurate. You make yourself unhappy." Wayne Dyer