For free, there is of course the fountains at Bellagio, the shows in The Forum, the battle in the lagoon in front of Treasure Island (get there early).
Lots of shows, and lots of really, really good food with prices all over the place.
But of course, there are the my favorites, the table games. Complementary drinks while you play blackjack, roulette, craps, etc. If you factor in how you are doing at the table, the drinks range from free to $400 each.
For just wandering around, love the Cosmopolitan (the hotel, not the drink: blech).