I'm going to add my vote for a trip to CO. Unlike anywhere you've listed where you've travelled. The kids will be awestruck by what they see. My pick for teens is the Salida area. Our teenage nieces were born and live near Vail. They ski, hike, mountain bike, etc. in that area, yet Salida may be their favorite summer time camping area. Great rafting, ghost towns, jeep rentals, biking, etc. etc. This is one of those trips your kids will always remember, and they deserve just such an experience. You can get to Salida by taking US 50 out of Pueblo and you won't have to go over any mountain passes. Several hours closer than Durango.
The secret to taking longer trips is to find interesting stops midway coming and going. How about Red Rock Canyon SP in OK? Cool little area with a pool to play in and rocks to climb. Or Palo Duro Canyon SP. If you do the route through Amarillo, Capulin Mountain Volcanic Park is just off the highway and a good way to break up the trip.
If you just can't bring yourself to make that trip, how about NW Arkansas? Plenty of COE CG's around some great lakes. Beside the usual water activities, there's Hot Springs NP, Crystal Bridges Museum in Bentonville (free and worth a whole day), Pea Ridge National Military Park (important Civil War Battlefield), and Blanchard Springs Cavern (probably the most pristine cave I've even been in). The Talimena Scenic Drive get you up high for some great views. Garvin Woodland Gardens near Hot Springs has trails winding through their flowers.
I know I'm biased, but I'd still opt to let the kids see Colorado!