There are plenty of cheaper hotels outside the city that are near METRO stops. Riding into the city early in the day and returning when you get hot or tired seems to be reasonable.
You may find one with a nice pool. If you tire of the DC area head for the National Aquarium and Inner Harbor in Baltimore.
You don't say how you are coming to the DC area. I prefer flying into BWI or Reagan National or riding Amtrak. Dulles is my least favorite airport.

We are flying into Dulles!

Regards
Gard
http://gardkarlsen.com - trip reports and pictures

You'll be very well situated at the Washington Plaza. I gather from your post on hiking the stairs of the Washington monument that you aren't put off by walking.

You'll be a five odd minute walk to the metro (McPherson Square) and a 20 minute walk to the National Mall. Great advice, as usual, from bardo for restaurants.

The DC Circulator is a bus system with simplified routes to popular destinations. It's $1 a ride (free or close to free if you use the SmarTrip card) and one of the routes goes past your hotel on 14th Street. (http://www.dccirculator.com)

Dulles is the least convenient airport for transportation to downtown DC but there are numerous options for getting into the city. Build in plenty of extra time for your departing flight.

Hope you have great trip!

My wife and I stayed at a nice suite hotel near the convention center. It is a half block from the Metro.

www.eldonsuites.com/hotels-washington-dc.cfm