Well, based on info received have made the decision to go to the SW region, so looking at Gunnison onto ouray, telluride/durango along to monte vista and back up to leadville stopping in breckenridge/georgetown on way home.
Or something like this.

Am now re planning, as we feel less chance of hitting forests with beetle problem and wether might suite convertable as this is hubby's one wish for..

keep all advice coming..

OH,GOOD! As much as I love the Estes Park area, the SW corner of Colorado is my favorite. The drive through Ouray is one of the prettiest ever. Be sure to take a ride on the Durango-Silverton narrow gauge railroad. Spectacular route! And, you HAVE to go to Mesa Verde NP. Cut the rest of the trip short to add all of those in.

I usually continue on east and stop by Great Sand Dunes NP and then head back up north to Denver. One big loop of the state. My favorite way to "do" Colorado.

I think you'ved made a great choice. The Southwest part of Colorado is breath-taking. It will not disappoint you. Get a convertible so you can see it all!

If you go to Gunnison, I'd suggest you drive a bit further up the road to Crested Butte to spend the night. I also recommend you focus on the SW--Durango, Ouray, Silverton, Telluride, Mesa Verde, Montrose, etc.-- and skip Leadville, Breckenridge, Georgetown. You will do better enjoying the southwest, I think, if you don't spread your visit too thin.

September can be iffy, so bring layers, as we always say. Have fun planning your trip. Ask more questions, because Coloradans love to help visitors.
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Fret not, and add Aspen to your itinerary.

Or, instead, you could come to Oregon and Washington. Here you could visit the Olympic National Park, Mt. Rainier, Mt. Saint Helens, the Columbia River Gorge, the Oregon Coast, Vancouver, Whistler (2010 Olympics), Seattle--it's great.

I love Aspen, but I don't think it's possible with their SW plans. You can't drive from Crested Butte over the mountains to Aspen without four-wheeling it, I believe.
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thanks all,

keep suggestions coming, though we are sticking to SW area.

Being from the UK what might be cool to you hopefully is hot to us, so when you say this area can be iffy in Sept do you mean below 70/75 decrees? or lots of rain?

As mid 70's is warm to hot for us.

We are not hikers etc, so we are looking for nice towns to mooch around, lovely scenery and breathtaking drives.

am getting excitied..

Ann - Good choice. Do a search here for "DebitNM" - she lives in that area and has posted numerous suggestions, comments, recommendations, etc. Very helpful.

As to weather, it can be in the 70s or even 80s, but tends to cool down at night (we've been in the area when it's gotten in the 40s - 50s that time of year). IMO the weather is usually pretty pleasant, and don't think you'll feel uncomfortable.

We always say our Colorado weather is "iffy", to cover us in case we get a freak storm in any month when guests are here! My family teases me, because I'm heard saying "it isn't usually like this" when the weather gets weird.

By that, I mean you can expect nearly anything here, especially in the Spring and Fall. Subsequent days can be very warm and dry, then cool and wet. You just never know.

But, check the weather stats on September in the SW, to see what is the "norm". I think you have chosen a great month to visit Colorado, and a gorgeous area to focus on. More likely than not, you'll have dry, 70ish days, then cool 40ish nights, as dfr4848 says.
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I too think you'll have a more satisfying experience in the southwest part of the state. The beetle-kill in the area of Grand Lake, Winter Park, and even Kremmling is quite prolific. That doesn't stop me from going to those areas - but I'm not on a once-in-a-lifetime holiday either.