So today's on-the-fly experiment: Capillary action with celery and potato plants.
Inspiration from: Wonderplay, Too! Another book that's great to just flip through and find a quick and doable project right when you need it. (Also great for bigger projects or outings when you have more planning time).
For the potato plants, take a white or sweet potato, stud it with toothpicks so it's bottom rests in a clear container of water. This one will require a few days' observance as it sprouts roots and leaves. (We'll get back to you with our results). I like the book's suggestion to have your kids draw the changes they see in the potato. Keep enough water in the container to wet the bottom of the potato.
For the celery, make a fresh cut at the base, drop the stalks in food-colored water, wait a few hours and then observe. You should be able to see that the stalks and leaves have perked up and that the water has traveled up through the veins and into the leaves. After seeing our results, you may decide to try darker water and colors other than yellow and green (I wasn't really thinking when I did it).
This bottom picture compares a blue stalk and a yellow stalk. You can actually see the yellow in person, and a marked difference between it and the blue, but not as much in the picture.