Have you had the chance to visit 'The Making of Harry Potter' experience in London? If not, I highly recommend it! My wife and I went there when we first started dating, and it was one of the highlights of our trip. Sure, visiting The Wizarding World of Harry Potter theme park at Universal Studios was awesome too—riding the Hogwarts Express and drinking Butterbeer while walking down Diagon Alley was an amazing experience. But I still had a better time in London.
There's just nothing like seeing the actual props and costumes from the movies. Viewing the large-scale model of Hogwarts used for the sweeping shots of the wizarding school and walking right up to the Dursleys' house, complete with robotic owls on the roof—you just can't beat that. So, if you haven't had the chance to experience the behind-the-scenes tour, don't think about it—just buy the tickets!