Science project ideas?

Fertilizer! Buy a few types of fertilizer, a pack of seeds, some soil, and a dozen or so pots. Plant one seed in each pot. Make groups of 3-4 pots, and use a different fertilizer in each group. Take height measurements every day for two weeks. Average the measurements, and plot them on a graph. Significant points to compare are the number of days until germination (when you can see a green sprout above the soil) and height of the plants after the two weeks.

How about gum. Last year each of my four group members in science chewed a different brand of gum. We recorded how good of bubbles you could blow and how long the flavor lasted. Hope this helps!

Toothpaste? Have each person in your family use a different brand of toothpaste and take pictures every day.