Make paper boats & laminate them (or have the children make their own boats) punch a hole in the end & tie a string to it & tie a magnet to the other end of the string. Present different objects (paper, wood, plastic, metal) to the children & see which attracts the magnet & pulls the boat.
This activity is good for talking about boats. Take a plastic sandwich bag and fill it 3/4 with water and seal. Then have several sharp pencils ready, ask the children what will happen if you poke a pencil in the bag of water? Then put the pencils in if you do it fast enough and steady enough it will not leak. Surprise. You can put many many pencils in one bag.