Eldorado Canyon is one of Colorado's many state parks. It is known for its hiking trails, rock climbing opportunities and picnic areas along South Boulder Creek. For $8 you can purchase a day pass that will give you access to over 500 rock climbing routes, and 11 miles of hiking trails.
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.
Colorado claims four national parks, including the Great Sand Dunes. These natural sand dunes lie at the base of the Sangre de Cristo Mountains and are the largest sand dunes in North America. These sands are 750 feet tall!