Now I'm at my laptop I can type out a proper reply for you!
The biggest/ best advice I have for you is to take the first few days slowly. Not only will you be jetlagged (duh) but being so high, you might suffer from altitude sickness, so if that's likely take a few paracetamol with you for headaches, chug back water like there's no tomorrow and yes you will feel sluggish walking around.
Colorado in general:
I asked about Stephen King (I've also seen his house in Maine, many years ago now, he's definitely Bangor's biggest tourist attraction in Bangor!) is because The Stanley Hotel served as the inspiration for The Overlook in The Shining so it's a bit of a pilgrimage if you're a fan (you have to pay some money to go inside if you're not staying there) but most likely. It's somewhere between Boulder and the Rocky Mountain NP.
RMNP is brilliant if you love wildlife, because of the cooler temperatures and lush green meadows, you're almost guaranteed to see wild moose as well as loads of elk. It's the kind of park that let's you go in one way and out another and is totally definitely worth going through and checking out the other wildlife too. Bring warm clothes (I think parts of it are over 2 miles high, so even in summer it's still cold and snow on the ground) and get food from outside the park too. You can't stay in the park and you wouldn't want to camp in the cold either!
Boulder is such a lovely city - very upscale yet liberal and alternative, on every street corner there seems to be a vegan macro restaurant. I've only really been there to hang out with my parents and my sisters so we've not really done anything very touristy but it is lovely to stop by in if it's on your way anywhere.
Golden, just 20 miles outside of Denver has the grave of Buffalo Bill as its big tourist attraction and the main town has a couple of nice streets with nice sports bars for pizza and other good stuff to eat, as well as gorgeous presenty shops. Again, not sure if it's really worth going to as a destination but it's en route and you need to stretch your legs or grab a bite to eat, you could do a lot worse than Golden . The other big selling point is the Coors factory tour which personally I wouldn't bother with.
I've also just realised that ll of those places are north/ north west of Denver so you might not be heading that way.
South of Denver about 1.5/ 2 ish hours is Castle Rock has a great outlet shopping mall (if that's your bag) with a good mix of high end and more mainstream shops and it's on the way to Colorado Springs which has the Garden of the Gods state park, which most people rave about. Again the main street in town is quite nice and servicable for food and amenities.
Denver
The street 16th Street Mall goes from the main train station up to the state Capitol (photo op next to the mile high marker) and pretty much anything you want to see, do, eat or would need to buy (get sunscreen there, much better choice) is either on it or just off it. You won't need to drive anywhere really in downtown Denver so this would be a pretty good base to acclimatise yourself for a few days. If you're there in the week, the Civic Center park area will probably have loads of food trucks there for nearby office workers.
This article from the Grauniad (
https://www.theguardian.com/travel/2017 ... and-hotels) covers all the major bases and I can't fault any of its recommendations- the breakfast burrito I had at Illegal Pete's in 2014 is still something I think of often. I also loved the Denver Art Museum, the modern section in particular was brilliant. The Mint was kind of fun too, if only because it was free and I was poor. There's also a Cheesecake Factory downtown, which everyone seems to love.