I've written an article about a system for dungeon crawling. It works for group play, but I think it's especially suited for solo play.

The basics of it is that you have a "Tension" value that reflects the current danger level of the dungeon, and it affects the risk of random encounters, as well as the danger level of other dungeon events. Combat and other noisy activities causes Tension to go up, but finding secrets, disarming traps and accomplishing other tasks makes it go down.