I'm quite familiar with Nova/DC area! Honestly just hopping on the metro train is the way I found all the places.
Water Park near the metro Station
Hilton Hotel (only if you are renting a room)
National Cemetary (metro stop)
There is a beautiful cemetery that has a buddhist garden and a pond with a bridge on it. If you need directions to it just ask, but it has some potential for photoshoots!
Amtrak station (Just get off at king st station I believe and walk to the amtrack station there is a cool old tunnel there and the bathrooms are old, so it gives a good setting depending on what you're looking for)
I'm still working on exploring, but if your cosplay has to do with asian society, China town would be a start.