Bring multiple yo-yos. People do BST. If your not competing you don’t really have to prepare. You could bring food. But it depends on where the location is. Have fun
Vendors are hit or miss, people do BST all the time at contests, Pants are recommended but not required, bring as many yoyos as possible, maybe bring a water bottle or pack a sandwich but you should check if there are going to be places with food around there, most contests break for lunch. Hope you have fun!