Yes, you are absolutely allowed to bring a Stanley Cup or any other reusable vacuum-insulated water bottle into Disneyland and Disney California Adventure. In fact, it is highly encouraged to help you stay hydrated in the California heat. However, there are a few "security rules" to keep in mind: the cup must be empty of any alcoholic beverages, and it cannot be made of glass (though all standard Stanley Quenchers are stainless steel, so this is fine). At the security checkpoint, you may be asked to open the lid so the agent can see inside. Once in the park, you can refill your Stanley at the numerous filtered water stations located near most restrooms and in newer lands like Star Wars: Galaxy's Edge. You can also ask for free cups of ice water at any Quick Service restaurant to keep your drink cold all day. Be mindful that because Stanley Cups are quite large (especially the 40oz models), they might not fit in the standard mesh pouches on some older rides, so you may need to place it on the floor of the ride vehicle between your feet during the attraction.