Apart from the spa itself, and water, you will need to have a towel close at hand to place your feet onto or dry them.
If you want to use salts or oils make sure you have them arranged before you start so that you don’t end up looking for them with wet feet.
If you want to give your feet a pedicure at the same time, you will need all your equipment for that, such as nail clippers, nail files, and a pumice stone.
If you want to moisturize your feet when you finish, don’t forget to have your moisturizer handy.