Pedialite is a good choice (Wal-Mart or Target brand is cheaper).
You can also use lite salt, which is 1/2 sodium, 1/2 potassium and Natural Calm for magnesium, which also comes with calcium if desired.
I love the Natural Calm (by Natural Vitality). Keeps you regular, too!
Actually, you could make your own electrolyte drink with these.