Collect seeds inside, from fruits and vegetables you eat, and outside, from trees and flowers. Sunflower seeds are easy to collect and big enough to handle easily. Nuts from trees are especially fun to collect, look at, and talk about. Since some of these naturally found seeds are difficult to sprout, you may also want to use some dried beans or seeds from the grocery store for this activity.
Wrap several seeds of each variety loosely in paper towels that have been soaked in water. Place the paper towels in clear plastic cups. Remember to keep the paper towels constantly moist. Observe the seeds over a period of days to see any changes.
For an extension, try placing seeds in baggies instead of cups and place them in different locations; in a warm place, inside a dark drawer, and inside the refrigerator. What do you think will happen? You may be in for a surprise!