Wilderness Exchange is within walking distance of Union and they require you leave a bag with them when you go into the store, they may be willing to stash your bag for a few hours. No guarantee on that, but an option if you can't find anything else. They are only open 10am-6pm on Sundays, so you may have to wait around a bit or pick it up early depending on when you get there and will be leaving.
In schools without lockers, students are sometimes provided with two complete sets of textbooks, one set being kept at school for use in class, and the other being kept at home for referring to for homework, thus limiting the amount of heavy carrying that would otherwise be required without having lockers to store them in between classes.[1] However, research has shown an increase in the incidence of back injuries in some students, which has been directly attributed to the lack of lockers for storing books in, thus forcing students to spend more time carrying heavy loads of books in backpacks.
Now that you’ve scored yourself some Denver luggage storage, the next thing on the list is finding things to do and places to explore. If it’s a Monday, a lot of the amazing indoor attractions and restaurants will be closed, but there is still plenty to do on any day of the week! See a concert at the Red Rock Amphitheatre, marvel at the beauty of the Cathedral of the Immaculate Conception or take a hike in the famous Rocky Mountains. You won’t regret it!
Bicycle lockers are usually in outdoor locations near railway stations and the like where people may want to store bicycles securely. They are often banked together, with individual lockers shaped like an isosceles triangle for efficient and compact storage of a bicycle. This triangular shape permits the lockers to be grouped either in a radial pattern (with the sharpest points of the lockers together), or in a row in alternating orientations.

The storage lockers were a “pilot,” the kind of small test that city government frequently uses to test a new or controversial idea. The city offered up the lockers for individuals to use for months-long stretches. At the time, city officials warned that “misuse of the lockers, vandalism, or other unanticipated results,” could force them to cancel the project.