money (try to have 10 yen and 100 yen coins for lockers if needed)
a bag to put everything in
Other things to consider:
any other products that you use when/after bathing (e.g. razors, shaving cream, cleansers, comb/hairbrush etc.)
a small pouch for jewellery/accessories - some onsen contain minerals that will discolour metals - so its a good idea to remove jewellery/accessories before entering the water
a showercap and/or hair wrap/turban (you can buy packs of shower caps and hair wraps/turbans at Daiso)
a change of clothes - onsen are a great way to end a day of exploring and after you're freshly bathed your probably not going to want to put dirty clothes back on
Beppu Spaport - Beppu has an onsen stamp challenge to become an Onsen master/ambassador. When you attend a participating onsen in Beppu for the first time you will receive a stamp