I normally head to Denver 2x year during ski season. Avis has always been good to me there. Cars are clean, normally get an upgrade. I'm the opposite of you, when I reserve a mid-sized SUV, I hope, and usually get the full-size SUV. Helps with all the gear.

Can't help you on your rate questions. My experience is if the website takes the coupon, you shouldn't have any problems. Bring a printout just in case.