Thank you all for your advice.
I wasn't aware of the traffic at that time. My plan was to take an hotel at DCA airport (my flight is at 7.30 am the next morning) leave my luggage and then take a car to see the Capitol / White House / Jefferson and Washington Monuments from the outside. Car rental quotes around 40 USD (which is ok for where I am from ) but if I can't park easily I guess there is no use. Do you think thoses places are safe at night ?

I did the Air & Space Museum 4 years ago and I loved it, unfortunately I don't think my GF will find it the same way

Given all of your answers, I think I'll try the Metro or Uber if it's more conveniant
Thanks again for your help