The Denver Art Museum is fantastic, with a collection of art of the west that you will see nowhere else! The building is also stunning architecturally. And it's only two blocks from the State Capitol, another beautiful building where the tours (whether dome or non-dome) give you a great sense of the area's history and culture. And the Capitol is free!
And right next to the Art Museum is the Denver Public Library (10 West Fourteenth Avenue Parkway Denver, CO 80204), with its Western History Collection of books, manuscripts, maps, and artifacts, including a railroad collection. You can see a hint of the
photographs in the collection, but more is available at the Library on the 5th floor - and that is all offered at the great price of FREE. Back on a visit in the 1980s, I found a Union Pacific traveler's and settler's guide to the route between Chicago and Cheyenne, Wyoming, published about 1869.
When I lived in the area, it was easy for me to combine walking and the
free 16th Street Mall bus to get from both the old Union Station and the new temporary Amtrak station on one end, to the Civic Center Station [bus] on the other end of the Mall. From the bus station, it is a pretty walk through Civic Center Park to the Museum, Capitol, and Library buildings.
HOWEVER, you need to remember that at the mile-high elevation, you won't have quite so much oxygen to breath, so you'll get tired sooner than you normally do. And you'll tire sooner than a resident, who is acclimated, would. You may get a headache if you overexert. Plan your day accordingly.
And you need to remember that the air is dry, so you will need to hydrate by drinking a lot of fluids during your day in the city. Alcoholic drinks dry you out, and can make you much more tipsy than you expect at lower eleveations. Despite that, most Denverites would recommend Wynkoop's, which is on a corner [1634 18th Street ] near the old Union Station, as a pleasant watering hole. Wynkoop's is Denver's oldest brew pub, founded about 1988 by the current governor of Colorado and his partners.
Watch weather forecasts carefully. In April, you might enjoy 70+ sunshine; snow and strong wind is equally possible. On sunny days (most days), the daytime temperature can vary 40 degrees from the overnight temperature.