I'm talking watermelon once a half a year treatJust make SURE you know what foods are good for the type of tortoise you have. For example, Hermann's tortoises (and other Mediterranean species) can get very sick if they eat fruit like strawberries and watermelon. Their digestive system was NOT DESIGNED to handle fruit sugars.
But other tortoises (like Redfoots and American box turtles, I think) have digestive systems that CAN handle the fruit sugars, so fruit can be a good treat for them.