I usually start of the school year with a measuring activity. This year, I used Tom Kuntzleman's Mentos and Diet Coke experiment and had students use the data to do some graphing and analysis. This was a nice lead into our gas unit. I also made my own syphon coffee maker to demonstrate for my students.
My first experiment involves measuring the density of water. Each group of two kids is assigned a specific volume of water from 10 to 100 mLs on the tens. They simply measure the mass of an empty graduated cylinder and then add the water and find the mass again. Once they have their data they go around the room and find another group that has one of the volumes that they need and get the data from them and record their names. Once complete they generate a graph of the data and answer a few simple questions. The whole procedure can be completed in about 20 minutes.