Go beyond "that's cool" with toddler science experiments by introducing the scientific method.
First, ask a question, such as "What happens when we add vinegar to baking soda?" That part should be easy with a toddler around!
Then, help your toddler come up with a simple hypothesis (a guess about what will happen).
Perform the experiment and record your results. You can give your toddler a "science journal." Allow your toddler to "draw" what he sees and consider adding photographs, too.
Finally, draw a conclusion about the experiment. Was your hypothesis correct?