We also have been to Las Vegas over 25 times and I always look for something different to do.
Have you tried the Mystery Adventure, where you try to solve the mystery like a detective? We spent at least half a day at the Atomic Testing Museum where they have actual film strips of the tests they ran in the desert. Scoot City is a real blast where you drive your own "2 man scooter" in Red Rock Canyon. We have taken the dinner cruise on Lake Mead, that is really a different look at Hoover Dam. Don't bother with Bonnie Springs, as it is really a dump. Someone suggested Mt. Charleston, and that is such a contrast to Las Vegas, it's cool. Valley of Fire is way better than Red Rock, in my opinion. If you haven't previewed a yet to be released TV show, go to the CBS testing exhibit in the MGM and sign up. It is pretty cool to be able to give your opinion on a new show and it is free. Only takes an hour or two.
The pinball museum/hall of fame is fun, especially if you played them as a kid. If you are a gun fan, go to the Gun Store and shoot your favorite weapon.
These are just a few of the less obvious thnigs to do. Hope you find something new to do this time around.