Washington DC has a big lawn in its center (but, like, really big. I'd say half an hour at least (walking) from one end to the other). And lots of museums around it. Historical museums (the indian american museum, the american history museum, the african american museum...), scientific ones (the natural sciences museum, the museum of air and space, and something that looks like a greenhouse), and artistic ones (the open air sculpture museum and probably some other ones). And they're all free.
It's really nice.

(they also have a shitload of veteran's monuments, but who cares about those?)

I happen to have spent my whole three days there at the american indian museum (you've got to make some choices), but having lots of free musuems is awesome. It also lets you only spend a few hours in a museum, and just come back another day.