Look for a vintage steel rigid Trek 850, 930, 950 or Specialized Hardrock or Rockhopper MTB. Usually available for $150 or less. These bikes are bulletproof and handle well. I own or have owned examples of each. STX and Deore are decent components.

I have the Muirwoods 29er from REI. It's a great boulevard cruiser, rock solid, but it feels huge compared to a MTB.