Nothing captures a child's imagination like combining two substances that when apart seem boring and plain, yet create a reaction when put together.
Nothing, however, is more frustrating to them than having to watch their teacher handle all the materials leaving them only able to watch from afar.
These projects involve materials that are not harmful for your students to handle (however, it is always good to remind them that science CAN BE dangerous and that if they want to participate they need to be very careful!)
Believe it or not, I didn't think of this one myself! This is the gold standard when it comes to elementary chemistry though. Collect a bunch of empty 20 oz soda bottles, and fill them about one third of the way up with baking soda. Give them to your students, and allow them to decorate the bottles. You can come up with whatever you want as far as decorating goes. Also, if you want, you can add red food coloring to the baking soda in the bottle, or to the vinegar you will be giving them. Once the bottles are decorated to look mountainous, give them the vinegar (doesn't take a lot to get the reaction.) Have your students wear gloves when we handle the vinegar, just because it makes them feel more like scientists. A safety goggle rule never hurts either. They pour the vinegar into the bottle (small funnels made of wrapped construction paper work well enough) and it foams over. Every one is happy. This, and the following project aim to have your students understand that things which do not seem probable can be possible. Ask them before you mix the two what they think will happen. It doesn't make sense to them, but they will love it.
MENTOS AND DIET COKE
This one is fun! It is also far easier than the above project, but proves to be a little more messy (urge your students NOT to try this at home, or you may end up with some unhappy parents!) Start by showing your students this video: http://www.youtube.com/watch?v=znoSaHwbHYg&feature=related . It is a fun and CLEAN look at what can be done using only Mentos and Diet Coke.
This project MUST BE DONE outside. Otherwise you will have a janitor's wrath to deal with. The most simple way to do this is get a two liter bottle of Diet Coke, and then to drop about 5-10 Mentos into it. Buy a piece of PVC piping that is about the same size as the Mentos and the 2 liter opening. Load it up, and let them slide down. It will create a spray of Diet Coke that can get pretty darn high!
Another thing you CAN do is drop 10 or so Mentos in to the 2 liter really quick, and then close the top. Shake the bottle up and throw it away from where everyone is standing. When it impacts the ground it will shoot up in the air like a rocket. NOTE: Although it is still pretty harmless, this could be dangerous if done in an irresponsible manner. Try it out on your own in your backyard and decide whether or not this is something you want to introduce to your students.