How to dispose of pure potassium
In a suggested lab, the RSC recommends using propan-2-ol:


  If there is any unused potassium remaining at the end of the experiment, remove it with the tweezers and place it in a beaker containing about 100 cm3 of propan-2-ol to dissolve away – it will fizz, giving off bubbles of hydrogen. When the fizzing has stopped, dispose of the resulting alkaline solution down the sink flushing it away with plenty of water. The acidic solution remaining from the experiment may also be disposed of down the sink flushing it away with plenty of water.


The same is recommended for sodium.