You're correct that the Strip doesn't do much to decorate externally -- Caesars Palace used to have a tree outside and a skating rink, but those are things of the past, and M Resort's not having a tree this year, either -- but you will find decorations inside.
The Botanical Garden and Conservatory at Bellagio is always one of the more striking holiday displays around town, with a huge tree and lots of fun and festive touches, like reindeer made from pecans and floral penguins. This year's display should be complete on Dec. 3 and it's free and open 24/7.
From November 26, the lobby of the Four Seasons will host its 12th annual Holiday Village, featuring 20 gingerbread "real estate" properties and a life-sized, working carousel.
At the other end of the Strip, we anticipate that Palazzo will also push the festive boat out -- last year's display featured a 16-foot tall polar bear mother and a 6-foot tall polar bear baby made entirely of a specially-developed white poinsettias, 25-foot silver-tip pine trees, 225,000 cranberries floating above a water feature, and snow!
Off the Strip, there are also several displays worth visiting. The Mystic Falls attraction at Sam's Town is always seasonally transformed into a "Holiday Wonderland" decorated with candy canes, lights, and festive vignettes, while the Hard Rock Tweeted excitedly last week about how they're decorations were going up already. The Fremont Street Experience downtown always has a big tree, too.
As far as non-casino displays are concerned, Springs Preserve does the eco-friendly thing with a massive LED display adorning its plants and trees, while Ethel M does the same with its cacti. The annual "Gift of Lights" display, now in its tenth year, has moved from its long-time home at Sunset Park this year and relocated to the Las Vegas Motor Speedway; admission's $15 per car (or $10 if you make a donation to Goodwill of Southern Nevada). Opportunity Village's Magical Forest is celebrating its 19th anniversary this year (General Admission is $9.75 for adults and $7.75 for children).
One other suggestion is to check out some of the neighborhood displays because, Vegas being Vegas, there are a lot of homes that really go to town with their lights and decorations. The Las Vegas Review-Journal publishes a "Holiday Home Tour" guide mid-December, pointing up the most festive houses and neighborhoods, as nominated by their neighbors (click the link for last year's feature).
These are the displays that we're aware of; if any new ones come to light, or if anyone out there knows of any we missed, let us know and we'll add them as "updates" to this answer.
