If there are any available slots for pavilions and events on the day of your visit, you can register (make a reservation) on the day 10 minutes after entering the Expo site using your smartphone or the On-Site Registration terminals located at the "Pavilion and Event On-Site Registration Centre" and six information centres around the Expo site.
On-site registration (reservation) is separate from the advance lottery reservations and available-slot advance reservations. You can make one on-site registration (reservation) at a time. You can register for another pavilion/event after visiting the first pavilion/event that you registered for.
The On-Site Registration terminals can be used without an Expo ID by scanning a printed QR code, paper tickets with a QR code, or the QR code on your smartphone. However, we recommended that you register an Expo ID before visiting so that you can make on-site registrations on your smartphone instead of using the terminals.

https://www.expo2025.or.jp/en/tickets-index/reservation/