Hi Julie,
magnets are a favourite of mine as well. I find it is vital to give the children time to play with the magnets and notice the patterns in what is attracted to a magnet and what is not. Since many metals that are easily found in a classroom contain iron, some children may erroneously conclude that all metals are attracted to magnets. I find it useful to have some coins, tin foil and some jewellery (silver or gold) to offer for testing.
Once children have played, ask them to generalise what they have found out (ie Only metals go to the magnet) – and from that make a starter question (Do all metals go to magnets?) and then investigate.
A fun next step is to make ‘bugs’ from pipecleaners (that have steel in their centres) and make a bug maze. Use the magnets under the table to pull the bug along the maze.