Weigh the aluminum in air, then in water. Measure the volume of water displaced, either using the calibration marks on the beaker or measuring the beaker diameter and the rise in water level. Calculate the value for the density and compare to the accepted textbook value. A spring balance is the easiest type to use for weighing the block. For purists, we have some which read in Newtons, but they have a minimum detectable mass of about 0.5 kg, so you will probably be stuck with a spring balance which reads in grams, unless you have a big piece of Al.