I don't want to give you the wrong impression. There are plenty of suburbs in the foothills, but the city (Denver) is on the plains. Access to the mountains varies depending on location in the metro area. Some suburbs snug right up against the mountains, while others are a good hour away by highway.
As far as trees, you're not going to find midwestern-style forests here, but there are plenty of places that have mature landscaping. It's just that massive leafy, shade trees aren't native. They have been planted over the course of many decades by homeowners and neighborhood associations. For example, in my neighborhood, the house behind me sports a cottonwood that is probably 50-feet tall. It was planted thirty years ago when the neighborhood was new, and it has thrived with good care, but it wouldn't have grown there naturally. In the foothills, the native vegetation leans toward conifers, with a sprinkling of aspen.
Here's a
link to an overhead photo of Denver that shows you just how flat it is.