Agreed on the transportation time to/from the airport.
More Central Business Dostrict/Tourist:
Larimer square has a lot of restaurants and coffee shops but if you go to 16th street mall (right on the same corner) there's a lot of shops (it's an outdoor mall), restaurants, and right behind it is the big blue bear. There's an I Love Colorado store on the second floor of the mall that displays artwork and small souvenirs from local artists so you can skip the tourist shops and buy something unique.
More of the Locals way:
There are red bicycle rental bike stations all over town. You can reserve in advance. Just hop on a bike and see the city really quick (head towards 15th and Platte to the Denver Beer Company and the highlands.
The highlands is a really cool place (not to be confused with highlands ranch). West highlands: 32nd and Lowell has boutiques, bars, and is more of the locals hangout. LoHi aka lower highlands is right above the Denver Beer Company and I suggest going to Little Man Ice Cream or try the best sandwich in town at Masterpiece Delicatessen. There's also Amatos brew house rooftop to see awesome views of Denver but I don't recommend eating here. over 7 years ago