I just returned from a trip to Colorado. At Sand Dunes NP I drove my SUV along the Medano Pass Primitive Road up to the Point of No Return. I think I got my best photos along this road. There is some loose sand and it's signed as a 4 wheel drive but I did see a lot of cars driving it. Just don't go past the Point of No Return sign. Otherwise if you don't want to drive it, park at the beginning and walk. There is nice green vegetation that looks good in the foreground with the dunes in the background. As stated late afternoon/sunset is good for photos since you get shadows on the dunes and mountains in the background. There are 2 pullouts by the Great Sand Dunes park sign, on the entrance road to the park, that are great for sunset shots. I also got some great photos in the morning. It takes awhile for the sun to get over the mountains so wait for some light to reach the dunes in the morning. By 10am the light becomes pretty flat.

The best photos of Garden Of The Gods are taken at sunrise and early morning. You could get more information at http://www.naturephotographers.net/i...bbs.cgi?a=vf10
You can read through the forum or become a member it's free and post a question. The people on this forum are very helpful.
The book A Guide to Colorado's Best Locations by Andy Cook is very good. It has pictures and direction to many good locations.