Hyperkalemia is always an easy brief MS4 talk. If you don't have a specific case to discuss, that is. It allows you to briefly discuss what are most common causes, physical exam, ECG changes, and a pretty straight forward treatment plan, some of which have validity, some not.
To set it apart from a very generic hyper K presentation, talk about the onset time and half life of the treatments (insulin, beta agonist, bicarb, lasix, sodium polystyrene, etc). Those are points people tend to not remember as much and is always a good refresher.
RustedFox's suggestion is good too. Something like aminita mushroom poisoning might be a fun one. Or to make it especially tricky, make it a 6 year old with down's syndrome and a congenital VSD, who was out playing in the yard and wolfed down a whole bunch of foxglove.