We just got back from CO. I used Expedia to search all of the rates. Thrifty ended up being the least expensive by more than 20%. BJ's travel also has discounts if you have a membership there. Thrifty still ended up less than that discount or AAA for us.
thrifty generally seems to be the cheapest. you can find a lot of discount codes for hertz though. i don't know if they check when you go to pick up the car to see if you're really allowed to use the code. for instance, 77694 in the CDP box on hertz's website will give you the USAA discount, which is very substantial. they also have $10 off a day on an suv right now if you look in their deals section.
Also, if you have a Costco membership you can take off another $30!
Thanks for this idea! I don't know why I never thought of it - my husband has a Costco membership through work. We don't use it much, but it came in handy for this, and it was about HALF of what I would have spent with the AAA discount for a 3.5 day SUV rental. There was a deal with Alamo for 25% off + $25 off. Sweet!
We ended up getting a van for about $91 per person (5 ppl) for 6 days, through Travelocity. Here is a great deal as well that we got through the same site, but it's not big enough for 5 ppl and all the gear and luggage: Through Travelociy, Avis has a 5-passenger Dodge Nitro (or similar) for $230.41 for 6 days. Pick up at Denver Airport. That is a sweet deal!!