One of the many draws to the Bellagio is their Conservatory and Botanical Gardens. There is one other like it that I know of, but this is the largest. It also changes from season to season, which just like Vegas outside, makes this something to keep coming back to see. It's like the Rose Parade in one room. It's a wonderful fantasy to walk through there at night with all the different sounds, smells, and colorful lighting.
One of the best things? It's free. Well, free to those who don't make deposits into the Bellagio slots. Anyway, this is one of those "must sees" if you're visiting Vegas.