There are tons of great spots to hike all over the foothills of the Front Range up to and including all the 14, 13 and 12ers!
More than happy to help, I have a couple favorites but maybe you could be more specific about difficulty. There are flat trails ALL over Denver, but all the good shit is in the mountains.
I enjoy quite a few in varying lengths in Conifer, Evergreen and Bailey when I can't get further away into another mountain range, all within 30-45min from i70 or 285/470.
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