First of all, how wonderful that you will get to experience DLR during the magical holiday season!! It is my absolute favorite time to be there, so I am always excited to hear when someone gets to experience it for the first time!! I will give you some links to threads that may be of interest to you.
Some of the other places you may want to visit in SoCal during the holidays are Rodeo Drive/Beverly Hills (the decorations are always beautiful during that time of year and it's fun to just drive around and soak in the atmosphere) and The Grove/Farmers Market outdoor shopping center (Farmers Market is a historical part of Los Angeles), which not only has some wonderful decorations and entertainment during the holidays, but is also great for celebrity hunting and people watching!!
To start you off, here is our delightful, wonderful, one-stop shop for all things Disneyland Resort-at-Christmastime thread (called, appropriately, "Disney at Christmas"). It is chock full of photos and info on decorations, seasonal food, special holiday photo ops, etc. at DLR during Christmastime:
http://www.disboards.com/showthread.php?t=2262171
Next, perhaps you might be interested in trying a character meal? Here is our dedicated character meal photo thread, which may help you decide which one to try:
http://www.disboards.com/showthread.php?t=2234435
I have stayed at all 3 DLR hotels and love them all for different reasons, but I, personally, love the GCH ambiance during the holidays, so check out our GCH Super-thread, courtesy of DmaxHawk, in case a GCH stay is something that interests you:
http://www.disboards.com/showthread.php?t=2396588
But many other DIS-ers love the DLH and other hotels during the holiday season, and for more economical purposes, so check out our hotel photo thread for other ideas:
http://www.disboards.com/showthread.php?t=1745747
If you do stay onsite at one of the 3 DLR hotels, you may want to consider the concierge lounge service, which, by all accounts, seems to be a good value for the money. Here is a thread with photos of all 3 concierge lounge offerings, courtesy of Pycees312:
http://www.disboards.com/showthread.php?t=2181963
In my own mega-trip report, which spans 37 years of pictures from visits to DLR, I have photos and/or recaps of DLR trips at Christmastime from 2007, 2008 and 2009 - and even holiday pictures from 1991, 1992, 1993, 1994, 2000 & 2001, and many other non-holiday trips as well.
http://www.disboards.com/showthread.php?t=1979268
As you are a WDW veteran, you may find these threads from fellow WDW vets/DIS-ers who either visited or are about to visit DLR for the first time useful, as they are personal experiences and observations shared of the differences between WDW and DLR:
Of course, be sure to read HydroGuy's essential, “A DLR Guide for WDW Vets”:
http://www.disboards.com/showthread.php?t=1162599
From RweTHEREyet: “DL Observations from a WDW Vet”:
http://www.disboards.com/showthread.php?t=2386987
A great Trip Report from Pmama, another WDW vet who made her first trip to DLR (as well as Universal Studios Hollywood and Sea World) last year and offered her photos, observations and comparisons - “Which Way to Disneyland?”:
http://www.disboards.com/showthread.php?t=2096377
From (jeff968): “How will DL and CA compare if you've been to orlando a dozen times?”:
http://www.disboards.com/showthread.php?t=2366196
From jfstorm: “Disney World Vets heading to Cali for the first time this year. Need Advice!!!!!”:
http://www.disboards.com/showthread.php?t=2365914
From Mommato4: “Disneyland vs Disneyworld”:
http://www.disboards.com/showthread.php?t=2359153
From Chris and Pooh: “Disneyland versus Disneyworld: Your views please”:
http://www.disboards.com/showthread.php?t=2233319
From GraceLuvsWDW: “DL Vets answer some ?s please...1 day trip soon”:
http://www.disboards.com/showthread.php?t=2222620
From THESCHULTZFIVE: “Disneyland disappointment”:
http://www.disboards.com/showthread.php?p=35296769&posted=1#post35296769
Enjoy the planning of your DLR holiday trip!!