The Colorado School of Mines in Golden, Colorado, has a great two-level museum of earth science, free to enter. Mainly it’s a collection of exotic and interesting minerals, with some fossils, mining equipment, and other items thrown in.
Younger kids probably won’t be too interested in looking at minerals for long stretches of time. However, the museum has a fun scavenger hunt set up where children can locate toy donkeys hidden in the displays. And downstairs children can walk through a fun “mine shaft” and complete a mineral puzzle. I think most kids would find the museum fun for at perhaps an hour or two. Older kids and adults with an interest in minerals may want to stay much longer.
Golden is a lovely town to walk around, with a creek and many pleasant statues along the streets and walkways. And various restaurants are within a twenty-minute walk of the museum. So the museum could become part of a fuller field trip.