I'm sure you'll want to check out lots of hotels as it is overwhelming, but if you are short on time - it's worth seeing the Venetian, the Bellagio (especially fountains at night), the Luxor...and there a couple of more, but I'm sure you'll see them all anyway.
Also, I highly recommend getting brunch or at least wanderig through the impressive Steve Wynn Hotel and Parasol Cafe. It is one of the newer hotels and is one of the classier (in Vegas terms) hotels to date.
Also, if you have time to see a show - O (is it still playing?) is a definite must for 1st timers.