Personally, given the short amount of time you will have, I would drive into the city. On a Saturday evening it won't be too busy. Come straight in on I-66 and go across the Roosevelt Bridge. This will dump you on Constitution Ave. Constitution runs along the north side of the Mall from the Lincoln to the ghetto with lots of great stuff in between. There is no need for you to go past the Capitol. It's not dangerous, but there just isn't anything to see. If you aren't doing a tour, I would park on 23rd, 22nd or 21st street and check out the Einstein, Lincoln, Vietnam and Korean memorials. Then drive or cab down to the Washington Monument area (park on 17th street) and see the Washington Monument and the WWII memorial. I think the museums close at 5:00, so if you want to see one of them, you might try that first, in which case I would drive directly there. At 4 or 5 on Sat I think you will find parking pretty easy.