There are several space centers around the country that allow visitors and/or offer tours. Here are a few that I found:
NASA Ames Exploration Center in California, south of San Francisco Bay
Goddard Space Flight Center, just east of Washington D.C. in Greenbelt, MD
Kennedy Space Center in Cape Canaveral, FL, east of Orlando
Marshall Space Flight Center, a musuem and home of Space Camp, in Huntsville, AL
Wallops Flight Facility, in Wallops Island, VA
Johnson Space Center, in Houston, TX (currently closed due to Hurricane Ike, but will be re-opening when they get power back on)
Another field trip idea is to visit your local planetarium or observatory and use their telescope to view the moon. You could also do this at home, but the telescopes at planetariums and observatories tend to allow for more detailed viewing.