One of my best day hikes out of three years spent in the US:

Swiftcurrent pass in Glacier national park, Montana.

Start the hike at the Many Glacier TH, enjoy grizzly tracks as you approach Redrock lake, don't miss the moose as you pass Bullhead lake then make your way up to Swiftcurrent pass by crossing numerous falls.
On the way down, take a break from the cold (hot?) weather at the Granite Park chalet before starting your descent toward the Logan Pass trailhead on the Highline trail. If you have enough energy, climb to a small "window" in the beautiful Garden wall to get a glimpse of Grinell glacier on the other side. Enjoy the numerous moutain goat and bighorn sheep sightings since your feet are beginning to hurt as you've now walked close to 16 miles.
End your day by hoping on the shuttle to go back to your car.

A small map here : http://www.nps.gov/glac/planyourvisit/upload/hiking_logan.swf

I need to come back on this board more often...