The Denver Mint offers free tours, but you need to make reservations quite a bit ahead of time.

This is a fascinating family tour - it helps to have kids old enough to understand the concept of bills and coins.  The tickets are free, but seem to be booked two months out, so reserve your tickets early.

http://www.usmint.gov/mint_tours/index.cfm?action=StartReservation (will launch new browser window)