The annual Superman Celebration occurs the second weekend of June each year, and brings thousands of people to Metropolis. Each year there are celebrity guests (2009 had Noel Neill, the original Lois Lane), games, and events. In 2008, the Guinness World record for the most people dressed as Superman was set at the Celebration. Kids will love it!
Metropolis is a small town, so if you plan to stay the whole time, you will need to book your hotel room in advance. If they are fully booked, look at staying in nearby Paducah, KY.