Most likely to lure you back for a return trip: Glacier National Park in Montana.

( nps.gov )

It's impossible to see everything on one trip. The park boasts 700 miles of nature trails and countless views of lakes, and, of course, glaciers. Plan on traveling the Going-to-the-Sun-Road, which will take you through the heart of the park and give you most of the views without the walking (and it's easier to escape a bear attack in your car. It's grizzly country, so if you're going to go hiking or camping, spring for the bear spray). The road is only open from mid-June until September, due to weather and construction, but the park itself is open year round. -- Jeff Cercone