I used magnets on the overhead to show attraction and repelling (electro-magnetic energy). It's fun to question kids if anything is stronger than gravity, although gravity is a hard concept at a second grade level, but of course, magnets are stronger (and we are to, for that matter, otherwise we couldn't stand up or jump).
On sound, I haven't been able to try this, but I read in E=MC^2, I can't recall the author, but it was in my local library in the adult science section, that if you are in a large room and talk on a cell phone to another person on the phone, that person will hear you on the phone before hearing your voice travel across the room; electro-magnetic energy travels faster than sound waves. I have a science demonstration event coming up soon, and I'm thinking of trying this with a speaker phone, but the obvious problem I haven't solved is that the sound waves from the speaker on the phone will be just as slow as my voice across the room.
The old cup-and-string telephone is still one of my favorites. Rubbing a sponge on the string makes a squeaking sound, too--don't put an ear on the other end, though, as it will be too loud. Connecting the string with buttons makes the best phone. Using a Slinky instead of a string makes a robotic sound to the person's voice.
This one is tricky to do. Stabilize a phonograph record on a pencil. I wrap masking tape just below the point to hold the record steady. If using a 33 1/3 RPM, a sharpened pencil that is then rubbed slightly dull can be a needle. A cone that doesn't trap the sound, experiment with various poster papers or use a Dixie cup, attached to the pencil will magnify the sound. This takes some doing, and isn't good for cooperative groups. If you get everything just right and give the record just the right spin, you will hear a bit of the record.
Light through a prism to see the 6 colors of light (indigo is no longer counted) is easy with an overhead. I'm trying to remember how I do this, but I think it works best held just above the flat writing part. I shine it on the ceiling. It's the easiest method I've found for controlling the spectrum I get.
Heat energy--this one's a little dangerous. I have everyone wear goggles and sit at their desks. Put a Coke bottle in boiling water. The bottle has a balloon on top. The heated air blows up the balloon. Let it cool, and the balloon sinks inside the bottle.