Buildings from Above
The Old Post Office Clock Tower is probably the greatest free view you can get in DC.
According to their website, the observation deck is 315 feet up and offers "the city's second highest vantage point after the Washington Monument."
From all four sides you can see the entire city in a way you've never viewed it before. If you live in the city, or even if you're just visiting, you have to go. I've been here for many years and had no idea this clock tower even existed.
But maybe go when it's not cold. Because we were forced to evacuate to the Congressional Bells to warm up after a short time.