I'm shocked at the amount of Colorado responses. I've taken the I-70 route to Glenwood Springs three times already this year and I love it every time. For all of you Colorado lovers out there:
The Great Sand Dunes National Park is one of my favorites. It's way out in BFE in south central colorado. Closest major city is Alamosa. It's the largest sand dune range in North America...and it's badass. As you approach the park you think, "Pfft, this is it?" But once you park and make your way to the dunes you realize how perfectly beautiful it all is. It's like your in the middle of the Sahara desert. It's pure, perfect sand and every day the dunes are different due to the swift winds at 7,500 feet. You can hike and get lost in the dunes and it looks like it goes on for miles. Bring a sled or snowboard-you'll thank me later.
Then you turn around and you're staring at 13,000 foot peaks still covered with snow. It's truly remarkable. I take my family every year during the first week of June. The runoff from the mountains is strong and it creates waves of alpine water through the dunes. I bring rafts for my boys. You hike, then cool off in the river. It's one of the most amazing/stark contrasts I've ever seen.
Then, go hike Zapata falls, and check out the Alligator farm (if you have kids). Then, at the end of the day, travel 10 minutes to Hooper Hot Springs. That area of Colorado has it all and I truly love it. You can do a google image search, but it won't do it justice. You need to be swallowed up by that environment to truly understand it. But, I'm telling you...late May/Early June is the ONLY time to go to get the best experience.
Did I mention the UFO tower and endless starry skies?