This is a simple activity that amazes any toddler. She may not be able to understand the actual concept behind floating but, remember, this is learning phase and every activity or every game imparts some amount of learning in the child.
The same activity may be repeated at a later age when the child's response will be different than the current one. She slowly starts to understand what floating is, then.
Materials needed: Water, ball(small)
Just put the ball in a mug of water and ask the child to push it down and move the hand out.
The ball moves up in an instance because of the concept of floating. Explain how the ball is moving up to your child.
Ask the child to repeat the same for quite a few times until she starts to understand what's happening and gets the feel of it.
My toddler exclaimed and laughed out loud as the ball was moving up.