This might give you some ideas:
Colorado has more acreage as designated wilderness than any other state except Alaska. Research the wildernesses before hiking them, though. Many (like Hunter-Fryingpan) generally only have large collections of out-and-back hikes without many thru-hike or loop opportunities.
I have GOT to +1 the San Juan and Weminuche wildernesses. That's an area on my wish list- specifically the CDT section through there. Honestly, I think that if I were in your situation that's probably what I would do. I worked out a 2-week route once that included some hot springs, but never got to try it.
At 10-12 miles/day you couldn't complete the CT in 30 days, though you might be able to at 16-17 miles/day. I have only done a couple of sections, but I'm convinced it's a great option nonetheless. I DREAM of thru-hiking it some day.
As others mentioned Great Sand Dunes is an incredibly interesting area in a way that is entirely different from any other national park- I have a trip report on it here on BPL. I'm actually very fond of it, and being just a couple of hours from my house doesn't hurt. But you could never spend 30 days there- it's pretty small, unless you add on a comprehensive trek through the Sangre de Cristos as well. Nonetheless if you wanted to hit several different areas instead of one big 30-day hike it would be a diverting contrast.
Lost Creek Wilderness also has the virtue of being close to my house and is indeed it's own unique thing, but nestled as it is nearly mid-way between Denver and Colorado Springs it can get "crowded." Not ridiculous mike most national parks, but if you're looking for true solitude and remoteness it isn't what you're looking for. If you want solitude stay farther west. But there are opportunities at Lost Creek to connect several loops into a large uber-hike.
Another thing to consider if you're breaking your 30-days into smaller chunks would be the Maroon Bells 4-pass hike. It is so beautiful that it looks Photoshopped. And there are numerous loops through other areas of the wilderness that you could connect together to make a larger more epic trek if you wanted to spend more time there. The only problem is its popularity- lots of other folks on the trails, particularly near Aspen. And if you want to start or stop at Aspen finding places to leave your car can be problematic. But on the plus side the bus takes you straight to the trailhead! It's actually easy (if rather expensive) to fly into Aspen and bus to the start point. But, Dear God it is beautiful!
Another spot on my wish list is Flat Tops Wilderness- you could spend forever in that place, just like the Weminuche. I worked out a route there too, once, but again the hike fell through. It is not the classic high-alpine hike people think of when they think of Colorado, though.
Wow, I can go on and on.
What month are you doing this? That might narrow things down.