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.
Centrally located between Downtown Denver and the D.T.C., Colorado Center was designed to be the central location for everything in your life. From a high-quality office environment to great restaurant and entertainment options, Colorado Center is a premier transit oriented development with three existing Class A Office Buildings. Tower III will complete the office component of the project, along with future Residential and additional Retail.