Stored my luggage for several hours at Lugden. Great location just across the street from Union Station. Spoke with the owner and he said he owns the parking spot so they are always there during the day. Prices were reasonable and I felt comfortable storing my bags there while I explored Denver. The owner was nice and gave me coupons for some local businesses.
There is a luggage storage service a short one minute walk from Union Station in downtown Denver - LUGDEN. You can make a reservation online - lugden.com, call - (303) 747-5600, or show up as a walk-in. They offer hourly, day, overnight and weekly bag storage. Great place to store your luggage if you're on a layover from Denver International Airport, or going to a concert, or headed to a Rockies game at Coors field and want to enjoy yourself without having to lug your bags around.