Samuel C Johnson IMAX Theater
Shows IMAX films all day long, mostly on natural history topics, such as dinosaurs and ocean life, in addition to some major blockbusters.
Tidal Basin Paddle Boats
Paddling around the Tidal Basin, admiring the monuments and the surrounding parks has got to be one of the best ways to laze about on the Mall. It's also one of the best ways to get sunburnt on the Mall, so bring lotion!
Sculpture Garden Ice-Skating Rink
A relaxing place just outside the National Gallery of Art to enjoy the fresh air, listen to music, and enjoy the surroundings, including a view of the National Archives building. Take a break from skating to enjoy hot cocoa or a meal at the Pavilion Café, next to the skating rink. Note that the sculpture garden itself closes M-Sa 5PM, Su 6PM, and access after that time is restricted to the ice rink only.
The Mall is also a public gathering space not just for protesters and pigeons, but also for locals and visitors alike who are more interested in a jog, a game of frisbee, or just a picnic out on a beautiful section of the nation's capital, in the company of monuments, history, and public art. Keep in mind, though, that a hot summer day, with the unbearable humidity, can be the worst time of the year for this—you will inevitably rush inside to the air conditioned sanctuary of the nearest museum.
Jazz-o-scope
Open air concerts by the center fountain under the Hirshhorn Museum are well attended and feature some great performers both local and international.