Experiment to see if objects float better in salt water than in fresh water.
Water, 4 containers, salt, paperclips, pens, small plastic ball, (items that will barely float), a stop watch and some paper to record results. This can be done as one large gorup or you can split it down to smaller groups but you will need extra equipment.
First fill all containers with water, next add 1 table spoon of salt to container 2, 2 table spoons of salt to container 3, 3 table spoons to container 4. Now add the items 1 at a time recording how long they stay afloat in container one. Dry of items and repeat again twice. Now move to container 2 and do the same again. Repeat with container 3 and 4. Group discuss findings.