Yes, you are absolutely allowed to bring a Stanley Cup (or any similar vacuum-insulated tumbler like a Yeti or Hydro Flask) into Disneyland and Disney California Adventure. Disney's park rules permit guests to bring their own reusable water bottles as long as they are not made of glass, which is strictly prohibited for safety reasons. A Stanley Cup is an excellent choice for a long day in the parks because it can keep your water cold for up to 24 hours and fits in most stroller cup holders. You can fill your cup at the numerous "bottle filling stations" located throughout the parks, such as those in Tomorrowland or Star Wars: Galaxy's Edge, or you can ask for a free cup of ice water at any quick-service dining location to replenish your drink. The only restriction to keep in mind is that you cannot bring a large cooler that exceeds 24" long x 15" wide x 18" high, but a standard handheld Stanley tumbler is perfectly fine. To make your life easier at security, it is often helpful to have the cup empty when you pass through the X-ray scanners, though most of the time, security will just ask you to open it if there is liquid inside to ensure it is not alcohol.