Small beach balls
Balloons (air and water)
Paper (crumpled/not crumpled)
Various toys (bath or just plain play time)
Water bottles filled with various amounts of liquids
Have three bins/tubs (clear see-through storage bins work well - or clean barf buckets) filled half full of water (to keep the water from overflowing) and label the bins: Sink, Float, Tricky (Both/Depends).
Have your scientist experiment with the objects and place the objects in their respective sink/float bin.
Concepts to discuss:
Density - mass of the object per volume. More "stuff" in a certain area = dense. A rock is the best example of something that is dense. It's not fluffy, there's little/no air pockets in dense rocks. Something filled with air (like a sponge) is considered less dense. Dense isn't necessarily "heavy" an example of something that is dense but light would be a bouncy ball.
Buoyancy - when an object floats, it is called buoyant, meaning the weight of the object is being held up by the forces the water puts on the object (in other words, the water hold the object up).
Neutral Buoyancy - Submarines are neutrally buoyant. They don't sink to the bottom of the ocean, and they also don't float on the top. It floats in the middle of the water. Can you make an object neutrally buoyant? Try experimenting with water bottles filled with various amounts of liquid. Who doesn't get excited about making your own submarine?