While you've gotten some good advice on what to see and do around Boulder, I'd also add a couple of smaller parks that are reachable as day trips such as Golden Gate Canyon State Park and Lory State Park. but this reply is going to be mostly about those 11 days you have for the drive back to Connecticut.

As Donna pointed out you've got plenty of time on the eastbound portion of your RoadTrip to see pretty much anything you want, and the things you might want to see don't have to be in a straight line back to Connecticut. So where I'd suggest you start is by picking out three or so sites or areas that you'd regret not having included and build your trip around getting to those. The Black Hills (with four or five National Parks or Monuments) have been mentioned, but you could also include the UP, the Great Smoky Mountains, the Outer Banks and/or the Shenandoah Valley and still fit it all within those 11 days.

As for routing, that will depend on which major natural sights you're going to visit, but in general, when I'm trying to do the sort of RoadTrip you seem to be envisioning, I tend to stick to semi-major US highways. I'll just give you a couple of examples of the types of roads I mean. Major east-west US highways include US-30, US-40 and US-50. But because they're 'major' highways, they tend to go through large cities such as Chicago (US-30), Indianapolis (US-40) or St. Louis (US-50). Interesting, but not scenic/natural. Whereas the semi-major US highways such as US-24, US-36, or US-62 tend to meander more, take you through smaller towns and generally provide a more relaxing drive.

So, your planning will be an iterative process where you first pick some major 'targets', see what roads best connect them, adjust your main targets and add smaller venues to visit, re-adjust your routes, and repeat until you've got a plan that suits you. The be willing to change on the fly as you travel. We can help with all of that, but the decisions are yours.

AZBuck