Muggles and wizards alike will recognize Platform 9 3/4 at King's Cross Station as the departure point for students on the Hogwarts Express.  Whether you are headed off to school for the first time, or just want to stop by for a quick photo op, a little schooling (but no magic) is required to find the fabled platform. 

Note: At this writing, King's Cross Station is undergoing a major renovation.  However, the platform made so famous by the Harry Potter tales (as told by J.K. Rowling), is still accessible.

The Gypsy’s Essentials

• Location: between platforms 8 and 9 (yes, that's correct) in London's King's Cross Station -- the train station, not the subway station.  They are connected, so follow the signs to the train station. 
• Price: free (see notes below)
• Who will love it?: Harry Potter fans of all ages -- wizard or otherwise
• Notes: you must have a ticket to get onto the train platforms.  However, if you have an Oyster Card, you can swipe it, go in to take your photo, and then leave without charge.  If you purchase an all-day pass, it will get you in as well (and it is the cheapest way to get around London via mass-transit). 

PS: Watch out for You-Know-Who!