We go into Denver every couple of months and looooove it! Ditto on what everyone's already said, also the Hammond's Candy factory is a must (yum, great behind the scenes tour, free) as is Celestial Seasonings teas (also free tour). The mint tour is ok but frankly a bit dry for the younger crowd, might be worth it to wait and do it when they are older. Still though, it's free and interesting.
Dinosaur ridge is a huge DEFINITELY, way cool to see the tracks! Also pay the $3 for the bus tour, the guides have loads more information and it's well worth it.
If I think of anymore I'll post, but Denver is just way too much fun and we absolutely love it, always something to do!