This is a link to San Diego Zoo where you can view live video feeds of animals in their enclosures, including elephants and pandas.
http://ictmagic.wikispaces.com/Science
A large collection of links to virtual "field trips". Not sure they all technically fall under the "field trip" classification, but it is a good collection of sites nonetheless. Covers all subject areas, but here are some examples for science: biomes, biomes of North America, clouds, constellations, Mars, microbe zoo, and many more.