Denver's botanic gardens are lovely and located near to the wedding venue.
The Rockies are a bad baseball team, but at least they play in a good ballpark. There are games at Coors Field every day from May 24 through May 29.
South Table, North Table, Green Mountain, and White Ranch are easily accessible from Denver. Or you can head a bit farther out of town and go for a hike at Staunton State Park, Kenosha Pass, or Brainard Lake (just to name a few).
Food, art, shopping. City stuff!