1. At King's Cross Station, there's a perfume ad that resembles the one Harry and Dumbledore stand by in Half-Blood Prince.
If you enter King's Cross Station from the London facade, you'll see it somewhere on your left. If you're taking the Hogwarts Express from Hogsmeade, it will be on your right.
2. At 12 Grimmauld Place, you can see Kreacher the House Elf peaking through the window every few minutes.
It's the second-story window on the right.
3. At the Magical Menagerie, there's a Crumple-Horned Snorkack, which is one of Luna's favorite creatures.
4. Also at the Magical Menagerie shop is a snake. If you stand outside the window, it will speak to you in Parseltongue, and then English.
5. At Wiseacre's Wizarding Equipment, there's a microscope that was used by Bill Weasley in the pre-show portion of the Gringotts ride.
You'll see it sitting on his desk.
6. Also in the pre-show of the Gringotts ride is a moving picture frame of the Weasleys in Egypt from Prisoner of Azkaban.
7. If you look on the second floor of Weasleys' Wizard Wheezes, you'll find the Peruvian Instant Darkness Powder.
8. At Borgin and Burkes is the Vanishing Cabinet. If you listen closely, you'll hear a bird chirping from inside like in Half-Blood Prince.
Even Draco Malfoy was amazed.
9. You'll also find the Hand of Glory from Chamber of Secrets.
"Insert a candle and it gives light only to the holder! Best friend of thieves and plunderers!"
10. Every 15 minutes or so, the dragon atop Gringotts Bank will breathe fire.
It will make a really loud sound first and then the fire comes. Have your cameras ready!
11. If you go to the Money Exchange, you can ask the Goblin any question and he'll respond.
12. If you dial 62442 (MAGIC) in the phone booth outside King's Cross, it will connect you to the Ministry of Magic.
13. With the interactive wands available, you can cast your own spells throughout Diagon Alley.
If you yell, "Metelojinx!" and move your wand the correct way, water will pour down from the umbrella above.
14. Right behind Weasleys' Wizard Wheezes is a statue of a mermaid as seen in the Hogwarts Lake from Goblet of Fire.
And she is pretty much just as terrifying.
15. Throughout the day there are various types of shows in Carkitt Market, like a re-creation of The Tale of the Three Brothers and a special performance by Celestina Warbeck.
16. If you hang by the Knight Bus, you might meet the conductor and Dre Head, which will result in a really interesting convo.
17. If you look in the window at Mr. Mullpepper's Apothecary, you'll see a set of shark teeth which is a tribute to the Jaws ride.
Diagon Alley took its place, hence the tribute.