My toddler daughter loves to play with water. I decided to try introduce her to the concepts of floating and sinking one day and she understood it immediately. I was shocked and very pleased! What a fun (and educational) way to pass the time!
All you need is water, a bucket, pool, tub, etc and then whatever random things you find around that float or sink. When we are outside this usually includes leaves, rocks, acorns, bugs, dirt, or pine cones. When we are in the bathtub it includes every toy and bottle of shampoo or body wash within reach.
Don't ever underestimate those toddler minds! I learn daily that my daughter is a sponge, taking absolutely everything in, and understanding the world around her in amazing ways.