If you like hiking, Catoctin Mountain Park is just down the road. Wolf Rock and Chimney Rock are both short hikes with a view. In Cunningham, you can take a short hike to the falls and kayak or canoe on Hunting Creek Lake. Cat Rock is Ashley's favorite hike in the area and has a fun rock scramble at the top.
Visit the historic town of Harpers Ferry, hike to Maryland Heights for a view of the Shenandoah and Potomac Rivers, and stop by Harpers Ferry Brewing for a drink. In the summer, you can also go tubing on the river with a local company like River Riders or Harpers Ferry Adventure Center.
There are tons of shops, restaurants, bars, and breweries in downtown Frederick. You can take a walk along the creek through town and into Baker Park.
If you're into Civil War history, there's a lot to see in Gettysburg, as well as shops and restaurants.
Frederick is about an hour from both DC and Baltimore. If you don't want to drive through DC, you can also park at Shady Grove and take the metro into the city.