If you're an architecture, design, or mid-century modern nerd, Frank LLoyd Wright's Fallingwater is nearby. I've only ever heard wonderful things about it, and it's about a 90-minute drive
Fallingwater is Frank Lloyd Wright’s crowning achievement in organic architecture and the American Institute of Architects’ “best all-time work of American architecture.” Located in rural southwestern Pennsylvania, 43 miles (69 km) southeast of Pittsburgh. The house was built partly over a...
fallingwater.org
If you're more adventure-minded, Ohiopyle, PA (3-hour drive?) is a small adventure down mecca. Whitewater rafting (which is relatively easy on the knees), hiking, mountain biking, etc
If you'd like to tour wine country, the Charlottesville, VA (2-hour drive) has a bunch of vineyards, and I think there's a couple companies that run bus tours, so you can drink most of the wine instead of spit it out
History-minded attractions in the area are plentiful. Mount Vernon, George Washington's manor, is a bikeable distance from DC. There should be a bus route there, but I don't know it off the top of my head. Gunston Hall is George Mason's (another founding father) house and about 30 minutes driving away from the city. Monticello is Thomas Jefferson's estate, 2 hours away close to Charlottesville, VA, and is fantastically preserved, with expansive grounds that have examples of gardens, kitches, stables, slave quarters, etc. Monticello in particular does a lot to reckon with the ugly side of Jefferson's history. Jamestown and Williamsburg are about 3 hours away. Jamestown was the first British colony in the future US, founded in 1604. Williamsburg has a massive historic downtown which usually is full of staff/volunteers living out historical roles on the weekends