We're heading to DC for Saturday night this weekend, coming in from the south via I-95. Our hotel is at Dupont Circle. Rather than pay the hotel rates for parking for one night, and to avoid driving in DC, we were considering just parking at a Metro station. The Franconia / Springfield would be easy to get to, and I believe that they do offer overnight parking.

What are the chances that there will actually be any spaces available?

Any other options for easy / cheap / safe parking coming in from that direction?

There will be spaces available on Sat. and Sat night at Franconia-Springfield Station. This is our home station adn we leave the car on Sat. and Sat night there all the time, and also have left it ovrnight on Sat nights when we go to NYC on the train (we take metro to Union Station and leave the car at F-S station).

Best of all, you do not have to pay on weekends, there is no person taking payment and no payment is required on weekends.

Great! Thanks!

There are also official multiday spots, plus lots of normal free spots, at the Huntington Metro station on the Yellow line. (There are two entrances to the Huntington parking decks; you would want the entrance from Huntington, not N. Kings Highway.) I don't think there are any particular advantages for you to choosing Huntington over Springfield, though.