Why not have a picnic at the Frederick Municipal Airport? Picnic tables outside the main terminal building give a clear view of the airplanes taxiing to and from the runways, taking off and landing. As long as the members of your group stay behind the fence separating you from the aircraft, you can stay as long as you like.
The restaurant inside the terminal building (which is open to the public) overlooks the airport, but you’ll need to purchase something to watch from in there. The Airways Frederick Inn hours of operation:
Breakfast: 7 – 11am M-F, Sat & Sun to 12:30
Lunch & Dinner: Daily from 11 until closing