Here's a very economic way to show how seeds grow and measure leaf and root growth, using a clear straw, wheat seed, paper towel marker, and ruler: http://firstgradefactory.blogspot.com/2011/03/can-seeds-grow-in-straw.html
Peas also grow rapidly. For an outdoor version, students can measure pea shoots daily as they climb poles. In the summer, use runner beans. Try varying water, soil, etc. and note the results!