2nd Grade:
Electrical Safety Games
http://www.rp-l.com/games.htm

3rd Grade:

Agriculture in the Classroom
http://www.agclassroom.org/kids/index.htm
Dr. Arbor Talks Trees
http://www.urbanext.uiuc.edu/trees3/01.html
Plant Nutrition
http://www.agr.state.nc.us/cyber/kidswrld/plant/
Plant Parts Salad
http://www.hhmi.org/coolscience/vegquiz/plantparts.html
Plants for Kids
http://www.plantsforkids.com/
The Great Corn Adventure
http://www.urbanext.uiuc.edu/corn/
The Great Plant Escape
http://www.urbanext.uiuc.edu/gpe/

4th Grade:
The Underground Railroad
http://teacher.scholastic.com/activities/bhistory/underground_railroad/plantation.htm