I know when most people say outdoor activities they mean activities outside of the urban center, but I would also urge you to consider doing some things within the city as well.
People don't just walk around the city for its own sake anymore. Either they are trying to get somewhere in a hurry or they take a car.
Denver has the state capitol buildings and the 16th street mall (which is a mile long, plenty of outdoors right there). It has the city park and the botanic gardens... you can wander around in the sunshine for hours without ever leaving the city's borders. And you'll save on fuel.
Plus so many people don't explore their own cities and it is a shame to be missing out on what you have right in your own backyard.