What's always intrigued me was air pressure and altitude. We all know air gets thinner as you go up, the inverse is true as well. A crater seven miles below "sea" level on Mars would have air pressure similar to the pressure of mountainous regions on Earth.

They'd probably still need a tent to block radiation, but they'd be livable breathable habitats. A few of these crater cities could dot the landscape while preserving the rest of Martian beauty, no terraforming required.