We did a really interesting canyon hike in Death Valley: Sidewinder Canyon. The canyon itself is a pretty desert wash with some nice views across the valley. The real attraction is the narrow slot canyons that enter the main wash. Some of these slots are so dark that you need a headlamp or flashlight in places. Definitely don't want to be here in wet weather.
This picture shows a doorway cut through an older fin. The slot originally went out and around (behind where I'm standing), then water cut a new channel through the bottom of the fin. We saw several places like this.
(Click on photo to enlarge).
Picasa album with more pictures.
GPS track and photos on EveryTrail.com
I got the beta on this hike from Jim Boone's web site Hiking Around Las Vegas. He has excellent information on day hikes in Lake Mead NRA, Death Valley, etc.