Things to see in Denver
State Capital, LoDo area for shopping/eating, baseball game!, the US Mint, Denver Art Museum, the Highlands neighborhood has nice shops and restaurants ( it's a trendy hood). Cherry Creek for shopping and the Tattered Cover bookstore!
If renting a car:
Golden can be a good few hours: Coors brewery, Buffalo Bill Cody museum & grave, school of mines, cute downtown,
Argo goldmine towards Evergreen, georgetown, and Morrison are cute western towns surrounded by hills and streams. And then there's Boulder, which is lovely. about 8 years ago
Tunnsey had some solid ideas. If the Rockies are in town And you like sports at all I would strongly recommend catching a baseball game at Coors Field. The ball park serves as the heart of the cool "LoDo" area of Denver with lots of restaurants and bars. My favorite thing to do before the game is go to Biker Jims for the most unique and in my opinion, best hotdogs ever.
I feel compelled to also paint a more complete picture of
The Casa Bonita restaurant mentioned in another answer. This is not a place know for the authenticity or even the quality of it's Mexican cuisine. This place is... An experience, more theme park than restaurant complete with loads of kitsch and extravagance. The restaurant features a three story waterfall with cliff divers that dive off it. Roaming mariachi bands. Little cheesy skits that have employed the same scripts and stunts since my childhood. And much more. It was even featured on an episode of South Park. So, while this may be a place to check out, let me reiterate that it is not for the food.
I hope you enjoy your visit to Denver, the weather should be awesome for you in September. about 8 years ago
Dan, what Tunnsey didn't tell you is that you get free beer if you visit Coors, in Golden...that is, if you're a drinker.
If you want to see Denver from far away and enjoy a stadium in the Dakota Hogback Foothills, visit Red Rocks Amphitheatre; it's open to the public if there are no events/concerts. You can hike Red Rock Trail, but be careful as I've seen adult diamond head rattlers.
Aspen about 8 years ago
Take highway 36 to Boulder in the mid morning and enjoy walking around pearl street for a couple hours. Continue west on 36 through Lyons and then into Estes Park (less than an hour drive if you are ok in the mountains). If you want more walking/shopping/food then Estes is a great place with the small mountain town feel. If you want some real views of mountains then just grab some food and head into Rocky Mountain National Park. Follow the signs to Trail Ridge Road, the highest paved road in North America with some of the best mountain and tundra views anywhere. There are elk, bighorn sheep and other wildlife all around. When you head back to Estes take highway 7 back to Lyons. This is called the scenic highway and following it in the late afternoon and reaching Lyons as the sun sets will give a fantastic ride home.
Spend an afternoon downtown, visit 16th street and then head to the LoDo area for pre Rockies baseball fun in any of the bars near the stadium (plan this around a night game) and then go to the game.
If you are a family guy especially, I'm not but still enjoy these, the natural history museum and the zoo are next door to each other. Just picking a few things you really want to see in each of them and doing both as a full day excursion is a great time.
If you want to try some hiking then head to Boulder in the early morning. There are more hikes into the hills from the outskirts of that town than any other place I've been. A good one that can be quick or drawn out and enjoyed is the Royal Arch hike which takes you up along one of the famous flat-irons to a natural arch with a cityscape view. about 8 years ago