If you're visiting for three days, I think the plastic SmarTrip cards area good $5 investment for one of the reasons mentioned here already and that is the paper ones are notorious for demagnetizing when in the vicinity of your cell phone. Not every Metro attendant will let you through on a dead paper card.

There are organized bus/trolley tours of the monuments at night. That might be a good option for you all especially now that the evenings are pleasant and cooler.

obxgirl, you're right, forgot about "stay" having that meaning, too--interesting! I actually think saying "stopping in G'town" to mean spending the night there makes more sense than saying "staying" there--because you're not staying, are you, you're just passing through.