Watch a movie
Many parks across the country offer free movies in the park. Bring some popcorn and something nice to sit on and settle in for some entertainment under the stars.
Image: Chicago Park District
Concerts in the Park
Music may be more your thing. Many opera companies and philharmonics schedule outdoor concerts. A quick search finds concerts in Portland, New York, and Madison as well as many other cities.
Either for dinner or afterwards for dessert. It's nice to just site at a sidewalk cafe and watch the world pass by.
Whether in your backyard or on the roof, head out when it's dark to look at the stars. Use the GoSkyWatch app to find constellations.
Just go for a walk
Get some fresh air and a bit of exercise.