You could spend over a week in RMNP alone. This state has so many photo opportunities it is crazy, and they change with the seasons. But, RMNP has something to shoot year round, you just have to be willing to hike up to some of the places. If you get a chance, look up J-Blake on here and look at some of his RMNP shots before sunrise. Some of the hikes take him several hours, but the photos appear to be worth the effort.
Personally, I was not all that impressed with Black Canyon. Don't get me wrong, the canyon itself is impressive, but the photo opportunities were not ones that would make my top 10 list for CO.
Garden of the Gods is great and in June there will be some green, which adds colors to the west side of the park for sunset shots. Sunrise is best on the east side of the park, and there are many different locations to shoot from. Some of the best are off of hiking trails around the park.
As already stated, the Great Sand Dunes are a must. The water should still be flowing pretty good in June. I could easily spend several days there alone getting sunrise, sunset, textures, contrasts, shapes, LOL, you could pretty much shoot all the elements at the Sand Dunes. It's a great place and there are 14ers around it that you can incorporate into some of your images.
San Juans and Red Mountain Pass is also full of scenery. That is another part of the state where you could spend weeks shooting and still not get it all.
Crested Butte is nice, but last year there was still a ton of snow and most hiking trails were closed. Even this one road had 3 feet of snow on it the first week of June last year. We had to down to Crested Butte South just to hike without snow everywhere. Had we remembered our snow shoes, it would have been a different story. But, it is still a great place. My favorite times of year to be in Crested Butte are late July during the Wildflower Festival or late Sept when the Aspens are turning.