If you guys are good with walking, I would take Metro in an walk the mall, monuments, etc. I've lived here for almost 10 years, and when I have visitors in town that want to see the city, that is how I do it. Driving into the city will add headache and frustration to your trip. Although Metro does have its drawbacks, it gives you the big city feel. It's pretty neat if you're coming from a place without a big rail system.
Also, Arlington is pretty cool and has some nightlife, however, I'm sure the hotels within walking distance to the Metro stops will be a little more expensive. If price is important but you still want to be in a safe area, try staying in Springfield and driving to the Springfield/Lorton metro stop on the Blue line. It's a short drive that's not all the scary. Definitely out of the congestion.
It's good that you're going in the spring. In the summer it can be pretty hot and miserable. Also know that during Cherry Blossom Festival, every high school in a 250 mile radius plans a bus trip to DC. It's going to be busy. I'd try to come right before the festival or right after. The date for the festival is set far in advance, but the National Park Service continually refines the peak bloom dates on the trees. They're really neat to see if you can make it in for that, but I'd avoid trying to come down the weekend of the festival unless you like big crowds.