HIKE. If you live in Colorado, as I do, you already know the meaning of hiking! There is no end to the number of hikes available, and they’re all free. All you really have to pay for is gas to get to the trail, and a few snacks and bottles of water for the journey. Don’t know of any hiking trails near you? Google the name of your city plus the word “tourist.” You’ll be amazed how many things there are to do, some of which you may not even be aware.
Source : http://www.theindependent.com/life/advice/great-date-ideas-for-couples-on-a-tight-budget/article_4b85dca4-1b49-11e8-8a0f-2300b43df04c.html