I have a layover and was taking a day tour. Is there lockers storage for a day at the Guangzhou airport

Yes, they have lockers for storage.
For visiting, the Yuexiu Park, the Sun Yat-Sen Memorial Hall, the Temple of Family Chen and Shamian Island are suggested to have a look. All these places can be reached by local subway lines. Then if you want to have taste of local snacks, the Shangxijie Pedestrain is suggested to go.