I try to change the objects in the Sink and Float activity often. Both my 4 yr old daughter and toddler loved this activity. The leaves that were used had been pressed in a phone book for a few days to keep them flat.
This is the preschooler version. I have three different types of leaves. The tongs are for removing the wet leaf from the water (she already knows how to use tongs) and the spoon is for scooping out the rocks. There is also a line on the water container, showing how full to fill it.
Fill the container to the line. Choose a leaf and gently place it in the water. Pause to observe that it floats.
One at a time, place a pebble onto the leaf (we used fingers for this).
Eventually the leaf will sink due to the weight. It is interesting to see how the different leaves hold different amounts of pebbles.
Using the tongs and spoon, remove the leaf and pebbles and place them on the towel to dry. This time invite the child to choose a leaf and have a turn. When finished, have the child watch you pour out the water, dry the container and dry the tray.
Toddler Version: We set up the towel and I filled a small pitcher with water (a few times) and my daughter poured the water into the container. I placed one leaf in the water and she placed the rest. We observed them floating.
I placed a pebble on a leaf and then another until it sunk. Once she saw me doing this she went right to work sinking each leaf with pebbles. She loved putting the pebbles into the water and onto the leaves. I stayed with her to watch and to make sure they didn't go into her mouth.
She equally enjoying scooping pebbles out and putting them onto the towel.