This is a site put out by the MoMA specifically for teens. There are discussions and activities for teens to participate in. This site exposes them to new art and gets them thinking about current topics in art.
Very simple, pre-K farm animal WebQuest. Probably not a stand alone "Field Trip" but a nice site for before or after a farm trip. Or just as a computer center in your pre-K class during a farm themed week.
Here are a few resources from the Baseball Hall of fame. I found some virtual field trips in math, but there are also some other things that might be interesting for other teachers too.