Visit the U.S. National Arboretum consisting of two 3.5 mile loops. You can option to do one or both loops as the first loop takes you back to the parking lot. This hike offers a very botanically diverse scenery with a wide variety of different trees, bushes, etc. The terrain is mostly flat with paved trails and easy to hike.
Directions from Baltimore: Balt-Wash Parkway (Rt-295) -> NY Ave toward DC. Note: NY Ave is 7 miles past Washington's Beltway (Rt 495). On New York Ave stay in the far left lane. Within a mile, make a left at Bladensburg Rd and in 4 Blocks another left at R St. Go two blocks to the entrance of the Arboretum parking lot.