Despite the fact that everything costs more in Vegas, there are a few remnants of free. You can watch the Bellagio fountains without paying (for anything but the trip itself.) You can watch the Mirage volcano and TI's pirate show for free. You can see the lion exhibit at MGM grand for free.
One of the prettiest freebies has got to be the gardens at the Bellagio. Many people never make it past the Dale Chihuly glass flower ceiling in the lobby, (and rightly so), but if they just pass through the lobby, they'll get a nice surprise. The Bellagio gardens change with the season. I've seen them at Halloween (Harvest), Chinese New Year, with the U.S. monuments theme and now with the spring theme.
This year it was ladybugs and snails made of flowers, and an enclosed butterfly garden. I've put together a photo slide show for you to enjoy (plus a couple of glass flower statues).