1) X-files UFO experiment/bike dynamo: a current flowing through a wire creates a magnetic field . The magnetic field produces a force that acts on a neighbouring charge e.g. wires parallel to each other carrying current in the same direction will attract each other.
Magnetic levitation (http://tesladownunder.com) involves lifting a coil off a copper plate. Attaching a coil to an AC circuit allows current to run through the many turns, creating a magnetic field (which is proportional to the number of turns). This produces a force on the copper plate, and the coil levitates i.e. 'anti-gravity'.
Similar experiment: create a dynamo. Moving a magnet in a coil of wire induces a current (Faraday's law) which can then power an LED. Can make something to move a magnet (similar to a bike wheel).
2) Lungs and CO2: linking global warming and how our lungs work. Using the idea of sodium hydroxide trees to reduce CO2. Breating into a solution of sodium hydroxide and bromothymol blue powder causes the liquid to turn from blue to green to yellow as it becomes more acidic. This can be done while at rest and straight after exercise to show the different levels of CO2 we produce.
3) Microbiology: this is very simple and I did it in year 6 - find out who has the dirtiest hands! Everybody presses their finger into a petri dish containing agar, and the bacteria are left to grow for x hours. This can be seen as a milky area in the dish. Kids who had washed their hands before the experiment showed the least amount of bacteria.
4) Design a boat: Thsi is also something I enjoyed doing in Junior school. We had to get into groups of 2 or 3 and build a boat out of basic materials that will carry the most weights in a fixed volume of water.
5) Build a missile: maybe make a rotating wooden disk connected to a DC motor (in a box) to represent the Earth and the children have to make a catapulting device and a sturdy missile to be launched at the right time from the rotating Earth to hit a satellite (if you're Chinese) or an 'asteroid' (if you're Bruce Willis). Lots of physics and a bit of astronomy to play with.
6) Create the best marble run: something easy and fun to do at the end of the day.