Right as you get to Main Street, you'll see the GINORMOUS Christmas tree with bigger than life decorations.
Then as you look down Main Street, you see the garlands going all down the street:
And the buildings on Main Street are all decked out too!
Over in the Big Thunder Ranch area (between Frontierland and Fantasyland), they have a cute little Santa's Village set up. The decorations there are just lovely - there are lots of great picture spots!
Then, after dark, you absolutely MUST go over to It's A Small World. Wait until the clock chimes. They have a wonderful light show that I didn't know existed. Here is some video of it (taken a couple of years ago - the first time I saw it).
Definitely take the time to ride on the "Storybook Land Canal Boats". It's the cutest thing ever. There are miniatures of houses from all sorts of Disney stories - and before the holidays, the fairies come out and decorate! Almost all those little houses have adorable miniature Christmas decorations. Sooooo fun! Aladdin's house was the only one that wasn't decorated. We figured they must be celebrating Ramadan.....
There is also a Holiday tour. One of Disneyland's knowledgeable tour guides takes you all over the park, pointing out special touches in the decorations that you may not have noticed (like that there's a Menorah in one of the windows on Main Street).
One of my favorite areas is New Orleans Square. The decorations are, of course, over-the-top, but beautiful. And while we were there, we saw the most amazing sunset:
They take you on the 2 rides that have special "holiday editions" - The Haunted Mansion, which is all made over with a Nightmare before Christmas theme (personally, I much prefer the normal version of Haunted Mansion) and It's a Small World where they show how Christmas is celebrated all over the world.
Then you get special seating for the Holiday parade, where they also give you a yummy gingerbread cookie and a toasty cup of hot chocolate! :-)
(By the way, if you are an annual passholder, or have an Auto Club membership, you can get a discount on the tour. We thought it was well worth doing).
And of course, after the parade, make sure to get yourself a good seat for the holiday fireworks spectacular (although I have to admit - fireworks choreographed to "Silent Night" is just odd...)
Oh! And how can I forget about the Castle!!!! It's my favorite part, naturally!!! During the day, the castle is covered in snow and icicles.
But at night.... WOW!!! The lights come on and the magic happens. It's BEAUTIFUL!
Disneyland's holiday decorations are definitely a festive way to start off the Christmas season! Early in November and early January (before Thanksgiving and after Christmas, but before they take the decorations down) are the best, least crowded times to go. They do have special Thanksgiving Day and Christmas day meal opportunities if you plan to be there during the actual holidays. I've never done that, but I'm guessing they'd do a great job.