Baltimore, IMHO,has the least . . . Unfortunately, it is best to keep to the tourist areas.
Hey! I resent the implication here
Baltimore is no more unsafe than any other big city, especially in the daytime.
If you want to get away from the tourist areas, several MTA bus routes will take you up to Charles Village with John Hopkins Homewood Campus (not the medical school/hospital) and Baltimore Museum of Art. Student type neighborhood, few tourists.
You can take the light rail to Falls road, and then walk up the boardwalk at the north end of the station to Lake Roland Park, for your nature fix; picnic tables and lots of hiking trails.
Walk along the Harbor Promenade to Canton and Patterson Park to get away from the tourists.
The tourist areas are nice, too. You can take the water taxi from the Inner Harbor to Ft. McHenry, which has the history, plus a nice view of the harbor.
You can take the charm City Circulator bus (free) from the Inner Harbor to the B&O Railroad Museum.
OK, so Baltimore doesn't have quite as much as New York or Washington, but there's a lot of good stuff here. And one can always take the MARC train to Washington or a quick ride up Amtrak to Philly for a day's outing there.