There are many treasures in the WWoHP for Potter fans. Some hidden, some not.
One of the things I tell fans going for the first time is to really take the time to look at the window displays(this is much easier closer to park closing during busier times of the year). The window displays really bring Hogsmeade to life, there are also a ton of hidden props from the books and films displayed in them. Keep in mind it is not just the windows at ground level that offer some magic.
Some of my favorites are:
This is an upstairs window display that you have to look up to experience. The instruments play on their own.
This is a side window hidden on the side of Dervish and Banges underneath the Owlery. The potions book might look familar to fans as belonging to a certain "Half Blood Prince"
There are also the Mandrakes display(which cry all day), Harry and Hermione's Yule Ball outfits in Gladrag's Wizarding Wear(keep an eye on the cat made of measuring tape below the dress), the quill that writes on it's own in Scrivenshafts ect. Besides the "magical" displays there are so many little details added to make Hogsmeade authentic such as signs promoting the "Nimbus 2000" and "Broomcare kits and accessories sold here". Everything is crafted down to the smallest of detail.
Inside the stores as well as the Three Broomsticks and the Hogshead Tavern are many little treasures as well. When moving between Honeydukes and Zonkos, right after you pass the display case of sweets, there is a sign on the wall that says "Look Up" and hanging from the ceiling you'll find Extendable Ears, if the shop is quiet and you listen closely you can hear whispered conversations. In all shops there are small displays such as Hogwarts trunks of select students, the Monster Book of Monsters in D&B, a wizards chest set with moving pieces ect. Read all the bottles behind the bar at the Hogshead and you'll find Ogdens Best Firewiskey, Pickled Poxie Eggs and other assorted wizarding goodies. In the 3B you'll not only see House Elf shadows but when you look upstairs you'll see a maid's cart that moves back and forth, hear doors open and close and hear voices of the guests in the rooms.
Outside in the village besides the window displays there are many other little touchs that add ambiance such as the Wizarding Wireless sign that hangs by the Hogshead, the clock tower on the Owlerly that at every hour on the hour an Owl pops out and hoots to mark the time, the Hogsmeade train schedule that displays the schedule and prices of the Hogwarts Express. The list goes on and on.
The best advice I can give is to just take the time to look at everything. It is more difficult to do in heavy crowds but if you can don't rush through the section to get from attraction to attraction, just wander around as best you can with the crowds and really get a look at things.
I have been terrified of seeing 'bloody mary' in a mirror ever since a slumber party in 4th grade. I still spook sometimes in a dark bathroom, and won't look in the mirror. I might just embarrass myself and DD if a face other than my own looks back at me. 
