Unless you've been living under a rock, you know about Bellagio's famed Conservatory. There are five displays during the year and they're all beautiful: Holiday, Lunar New Year, Spring, Summer, and Harvest. There are elements that remain the same for the displays year after year, but there are always new features as well.
Their website shows the dates of each display, so you know when to visit and to avoid the transition periods between displays (usually a week); very helpful.
It does tend to get REALLY crowded and makes it difficult to take photos or even enjoy. I recommend avoiding holidays and weekends. However, if you must visit during these high traffic times, come early in the morning (though you may catch them refreshing the display).
The conservatory is located adjacent to the front desk, so it's easiest to access from their parking garage. It is also close to the tram.