Make your own barometer. I am interested to see how well this works. I got the idea from http://www.parents.com/fun/games/educational/teach-about-weather-with-this-science-craft/?socsrc=pmmpin130627ffBarometer
This next one demonstrates air pressure and the fact that hot air takes up less space than cold. The bottle has hot water. Place a balloon over the top and put in a bowl of ice water. I got the idea from http://savvyandsleek.blogspot.com/2011/08/kid-craft-science-experiment.html?m=1
This demonstrates density. Fill a bottle with water. Add a thin later of oil. Add a few drops of food coloring. Sprinkle with salt. The salt is more dense than the oil so is moves through it carrying the color and a drop of oil with it. The oil bobs back up to the top. I got this idea from http://www.kiwicrate.com/projects/Salt-Volcano/2331
This year I am planning on having the kids make blocks from milk cartons. We will stuff them with paper from the recycle bin. Since we have about 20 kids and drink milk every day we will end up with a lot of blocks!