Iron filings are a classic and fun way to show magnetic fields. The new and inexpensive magnetic viewing film (you can get it from ForceField) is a great way to see the same thing without the time and mess.
From left to right I add more filings to the mix to slowly fill out the shape of the magnetic field lines.
The finished product in closeups.
A set of nine ring shaped magnets.
A very powerfull rectangular NiB magnet from a hard drive.
To get field lines on this one I would have to cover an entire sheet of paper (or more) due to the field line density anywhere near this thing.