We had many stations (probably too many considering we only had 3 adults helping) and they were able to rotate from table to table.
For the Density Rainbow they added different liquids together (for this dish soap, body wash, oil, and water with food coloring). Each time they did it, the ingredients settled in the same order.
They cleaned pennies with 1/4 cup vinegar + 1 tsp of salt. Rinse after shaking in the mixture so they don't turn green.
They also got to:
Fill a balloon using baking soda and vinegar.
Messy fun was had by all :D