Depends on what is easy for you. Plain table salt will work too. Or Morton's Lite salt if you want more than just sodium.

I use Hammer Nutritions Endurolyte powder since I put mix in my bottles before I leave the house and sometime carry a couple containers with premeasured mix for times I have to fill up along the way. Others here like their Endurolyte capsules which also come in a pack of 4 and fit nicely in your pocket or bag. And they also have fizz drops you can add to your water or mix.

https://hammernutrition.com/collections/electrolytes