Bring a large board like a 2-by-4 to class. Stack a pile of books and center the board on it. Set another book or heavy object on one end of the board. Have the students try pressing on the other end to raise the book. Move the board so that the arm of the lever with the book is longer, then move it so that the arm that you press is longer. Ask students which way is easiest and discuss why this is so.
Bring a set of Hot Wheels cars and tracks to class. Be sure to bring a wide variety of tracks, with low inclines and high inclines. Have the students set up a track with variations of inclines. Let a car go down the track and ask the students to observe when the car goes faster and slower. You can also let them build their own tracks out of cardboard, books, rulers, plastic piping or other materials you have in the classroom.
Set up a pulley by hammering a thin nail to a piece of wood and slipping a thread spool over it (make sure the pulley can still move but won't fall off). Set the pulley over the edge of a table. Create a loop at the end of a piece of string and set the string over the pulley. Attach some washers or a 10 to 50 kg weight to one end of the string and use the pulley to lift it. Attach a spring scale to the other end of the string to measure how much force it takes, then measure how much force it takes to lift the weight with the string just going over the nail.
Have students bring in materials to construct their own cars, water wheels or pinwheels. To make a water wheel, make eight equally spaced cuts in an aluminum pie tin, from the edge to the bottom. Twist the cut edges to make blades -- it should look like a windmill. Poke a hole in the center and insert a pencil. Set it over a wide pan and pour a pitcher of water over it. A pinwheel has a similar structure, but it can be made with construction paper and a straw.