One of our favorite rides in both Universal Parks, and especially the Wizarding World of Harry Potter is the Hogwarts Express. This train allows you to travel between both Diagon Alley and Hogsmeade without having to exit one park, and walk all the way to the other park. While inside you get to enjoy the country side while sitting in a very detailed replica of the Hogwarts Express car. It also allows you to visit both Harry Potter parks without having to leave the Wizarding World.
In order to ride the train however, you have to make sure you have a park to park pass to go between the two theme parks. The train is air conditioned and makes a great way to escape the heat for a few minutes. We have also found that buying some wizarding candy before riding the train makes it even more fun. Keep you mind you should ride it both ways, as it is different each direction.