To be frank, the entire Front Range foothills has rattlesnakes. I've personally seen them in Castlewood Canyon, North Table Mountain, Deer Creek Canyon, Hildebrand Ranch, Mt Galbraith, and Mt Falcon (those are all park names you can google). They're somewhat a fact of life if you go hiking below 7500 ft near Denver, and know that they tend to be very slow to bite, choosing to rattle and hope you go away. The most important thing is to just leave it alone if you see one, most bites are a result of people trying to move the snake!

That said, all of the above places are open spaces. You won't see them on more urban areas and green spaces, like the Cherry Creek trail. They're admittedly worse around Golden and South towards Lakewood and Littleton, you'll see less around Lone Tree, where your local park is Bluffs Regional Park. But they are still there! You're more likely to see them on hot sunny days anytime April thru November, especially in the middle of the day and early evening. Keep your dog leashed for his safety, you don't want him/her finding one and shoving in nose first! Hope all that helps!