There are coupon codes for Enterprise that waive the underage fee and may get you a discount. Check the car rental threads for those.

There are two unique Enterprise locations in Bethesda, MD - accessible with a walk from Metro - that might make your rental a little more fun.

Enterprise on Wisconsin Ave handles hybrid rentals and usually has plenty of Toyota Prius'

Enterprise on Bethesda Ave partners with Euro Motorcars and has nice Mercedes for rent. There is a yelp account here.

Make sure you call ahead to find out and reserve particular cars. The sales guys are pretty helpful IME.