Long challenging day hikes

Would be my vote. Loads of great dayhikes.

In TNP, the Cascade-Paintbrush loop is fantastic. Plenty of other 20+ mile loops are possible.

In Yellowstone, Specimen Ridge and Mt. Sheridan are the most obvious candidates. Both world class. Specimen requires a hitch back at the end of the day.

Another Yellowstone option is to start in Gardiner, hike up the road a bit, then up through the Rescue Creek Trail, then return via the Black Canyon of the Yellowstone.

Check out Rocky Mountain National Park. So many options for you. Also, you could base in Boulder and/or Estes Park and/or Grand Lake–all three towns offer different things and all are enjoyable (I was born and raised in CO and lived in Boulder for around 8 years).

There are countless day hikes/overnighters in the Crested Butte, Colorado area. Conundrum Hot Springs is a favorite, but popular, so go on a weekday.

The earlier post about Grand Teton and Yellowstone is right on target – best backpacking area I've ever been to, with plenty of day hiking opportunities.

The area around Moab is a great alternative to the other wonderful suggestions, particularly if you are thinking during the winter or shoulder seasons. There is Arches, Canyonlands (Needles, Maze, Islands in the Sky), Slickrock (mtn.biking), and the Colorado River.
http://www.flickr.com/photos/tomclark1/sets/72157602580188376/

The Three Sisters Wilderness area in Oregon has glaciers, 10,000 ft volcanoes, lava flows, hemlock forests, hot springs, waterfalls, etc. http://www.flickr.com/photos/tomclark1/sets/72157602580188376/

Thanks for all the ideas. Just did a fun 18 miles on the Jordan river pathway this weekend here in Michigan.