We have been a few (three) times:
- definitely look around all the other hotels casinos (including the Bellagio with it's botanical conservatory, the Venettian with it's gondolas and little Venice, Paris, the shopping mall at the Aladdin, the Luxor, the inside of New York New York etc).
- Go up the stratosphere tower for the view (as someone else mentioned): there is also a small roller-coaster and a couple of other "thrill" rides at the top of the stratosphere.
- There is quite a big rollercoaater wrapped around New York New York.
- I think that there are some circusy things at Circus Circus.
- I believe that there is still a Star Trek ride/exhibition at the Hilton (but that is a couple of blocks from the main strip).
- As Carol mentioned, the "free shows" (the pirate ship show at Treasure Island, the Volcano at Mirage, the Bellagio musical fountains etc).
- There is an aquarian of some sort at Manadalay Bay.
- There is a water park in Las Vegas (Wet and Wild I think)... no idea how good it is though.
- Catch some of the shows (
http://www.lvol.com/lvoleg/lvshows.html might provide some insight into what is on when you are there and will allow you to pre-book). We say a "cirque de soliel" show at Treasure Island (although there is another one at the Bellagio too). You might find other interesting acts on tour in Las Vegas (Elton John did a spell there a couple of years back...).
- Go out to Hoover Dam and see the Dam and learn the history about it.
- A Carol said, take a tour to the Grand Canyon: you could do an overnight trip or do a helicopter tour out there and back.
And don't forget to check out some of the great places to eat. Las Vegas is famous for its buffets (personally I loved the Bellagio dinner buffet and their brunch buffet and the Rio Seafood buffet (not their normal buffet which isn't supposed to be too great, they have a seperate seafood buffet restaurant). There is also a HUGE range of different dining experience from "cheap all you can eat", to "amazingly expensive" and everything in between.
I tend to think of Las Vegas as the "grown-up Disney Woirld" - it is heavily "fantasy" themed, but the place is directed at adults (proper meals, gambling, shopping, shows etc) and not children.
To be honest, it might be worth you getting a "rough guide" or something to read through before you go... there is so much to do there.
You will have a great time.
Boo