I was thinking you could dome a large canyon with plain old glass and steel. Pump up the atmosphere inside and you'd be able to farm with day light and have a huge "outdoor" public space for people to enjoy. Buildings could be built into the canyon walls.
I'm not sure if structures like that can be made that would also provide adequate protection from radiation. My default assumption is that any early Mars colony would be doing almost everything underground, because that's the safest place to be.
