That's a very big question. I've been to Vegas a few times on business and then insisted on taking the family there. I thought that it was one of those 'once in a lifetime must do' places. Very artificial but very special.
My suggestions of 'must do' items ..
1. Bellagio Fountains … stunning fountains to music … they used to do an Andrea Bocelli piece that was awesome
2. Olive restaurant … but book in advance and demand a patio table … best of both worlds - good food and the Bellagio fountains … I have found that early evening is the best time.
3. Select one of the Cirque du Soleil shows … Ka is my favourite (at MGM) … my wife and my 26 year old son loved it.
4. Do a trip to the Grand Canyon South Rim … we took a light plane to the Grand Canyon Village and then a helicopter ride over the canyon … stunning views and we had a chance to look around the village too. The flight took us over the Hoover Dam … amazing perspective from the plane.
5. Walk through the big themed hotels … Ceasar's Palace ceilings are stunning; the gondolas in the Venetian are quite striking; Freemont Street light show - pretty amazing.
6. Have a 'limited' gamble … my wife loves a short spree on roulette