Our family has always been a major consumer of baking soda and vinegar, both of which are very cheap here, thankfully. The boys have had a lot of fun with just these two substances on and off for a long time. Now imagine everyone's excitement when Santa Claus brought a real chemistry set, complete with test tubes, pipettes, magnetic iron shavings, and chemicals, among other things. Together with the microscope that we bought in Italy last summer, we're really rockin' it now!
Notice the (almost) four year old's face - he - especially - loves every moment of it (he doesn't have to write a paper or a report on every science experiment we do), and asks to do science at any odd hour of the day.
I totally take advantage of this, and often set him up - so that I can work with the boys on specific subjects or projects - with a tray containing baking soda, soap, a little water, a few drops of food coloring, and then arm him with a spray bottle of vinegar. Spray a section of the tray, and colorful foam bubbles up. "Bubble, bubble, toil and trouble," he mumbles, as he enjoys himself for hours by the kitchen sink.