Seneca Creek Greenway Trail - walks around pond
South Germantown Park (Boyds MD) - paved
Black Hill Regional Park
Wheaton Regional
Lake Needwood
Capital Crescent - but this is super busy with rude people that will run you over with their bike and nobody says hi.
https://montgomeryparks.org/parks-trails/trails/page/2/#:~:text=Montgomery%20County%20Trails%201%20Paint%20Branch%20Trail%20This,8%20Upper%20Rock%20Creek%20Trail%20...%20More%20items
C and O Tow Path, I like to start at Riley Lock or Violet's lock, or Poolesville - it's a pretty ride if you take the backroads. Parking if free everywhere (I think) except Great Falls. It's way less crowded the further you drive.
https://www.canaltrust.org/plan/co-canal-towpath/