Colorado has four national parks and 42 state parks, offering a multitude of wonderful hiking trails. However, if you're looking for some free hiking places, here is a list of my recommendations. These are all easily accessible from Denver, and are ranked by difficulty.
Rocky Mountain National Park was established in 1915, and is one of the most popular national parks in America. In 2016 alone, 4,517,585 people visited the park. Visitors are drawn in by the mountains, the tundra, and the wildlife.