This is really appropriate for all ages. Have students donate pumpkins. As long as you have a variety of sizes, it works great. Use a digital scale to get the mass of each pumpkin and then have students make predictions about each pumpkin's ability to sink or float. Students should discuss their thinking.
Place each pumpkin in the water and have students observe, make predictions, write down data on the data sheet, and draw conclusions.
As an extension use the MATH CONNECTION PUMPKIN PLOP SHEET.