[MetalMan]

Discharge the pack completely (a charger set to discharge in NiMh mode), and then put a resistor with a small resistance on the pack when you discharged it as far as you can with a charger. A lightbulb would also work.
Leave the pack on with the resistor (or lightbulb) for a day or so, and when that's done, then you can put that pack in salt water. If you want to have some safe fun, you can pierce the cells while they're in the salt water :).

If you only use salt water to discharge the pack, there's a chance it won't actually be discharged by the time you throw it away. One of the tabs on the cell (I believe it's the negative nickel one) will most likely dissolve before the cell is completely discharged, rendering it dangerous still.