Activity: Harry Potter Studio Tour. Tube Stop: Euston, more details below. Cost: £43.00/adult, £35.00/kid Hours: 9-8.
Take the tube to London Euston Station. From there, take a train to Watford Junction. There's a shuttle bus that costs 2.50 each way that will take you to the WB Studio from the train station. The first bus is at 9:20 am. Try to make it to the station by then to have a less crowded experience. Expect to spend 3.5 hours here. It will probably last into the afternoon.
Though you've seen some filming locations throughout this guide, most of the movies were filmed on location at Warner Brother's Studios just outside of London. They've done an excellent job preserving the movie experience as well as the props. The tour is self-guided. There's a pretty affordable cafe on site. Make sure to taste the Butterbeer!
If you make it back to London in time, catch one more show before you leave! Extra points if it's The Cursed Child and you have the most Harry Potter day of all time.