Bathtub, 2 buckets or butter tubs, small objects that float (boats, wooden block, bar soap, sponge, plastic containers, etc.), small items that sink (rock, spoon, hair brush or comb, etc.)
Young children will notice that some things float and some sink. Ask your child to put all the things that float in one bucket and all the items that sink into the other.
As always, watch for things that have sharp edges or might be choke hazards. Do not use small objects for children under three.