Bouyancy with plasticine shaped different ways is fun and easy, we did recycling paper at that age as well. When my kids were 2 and 4 the oldest nce fruit juice ice cubes were softer than standard ones so we made ice blocks out of plain water, salt and suger solutions and smashed them with hammers to see which was hardest then melted some in the sun and chucked some on a barbeque grill.