Anywhere on the Eastern Shore is a great relaxing getaway from Baltimore. The great Northeast suburban sprawl hasn't jumped the Chesapeake yet - expect quiet country roads, lots of open water, and a very relaxing area.
From Baltimore, you can drive out I-270 and I-68 to the far western tip of Maryland, Garrett County, by Deep Creek Lake. This is well into the Allegheny mountains. Most of the recreation around here revolves around the lake, but there is also a downhill ski slope and lots of hiking and mountain biking trails.