walk the Bellagio lobby too. Its more impressive than the fountains.
see the Lions right in the casino at MGM. take a look at the "open sky" casino in Paris.
the Pirate show at TI is cheesy, but worth a few mins if you happen to be in the area at the time.
DON'T walk all the way down Strat. its a dump down there but if you hit that area, walk Circus Circus too
do a little research if there is stuff you are particularly in to. there are a lot of cool things (ancient Egyptian artifacts @ Luxor, cool aquarium at Mandalay Bay, knight doing combat at Excalibur, mob & Liberace museums, etc)
tons of great food, both high end and low. again, do some research but the above suggestion for the Bellagio buffet is spot on.
like others said, hit up Freemont for a few. down the block, just past the lightshow are zip lines, if you are inclined. if you are a poker player, spend some time playing at Binions when down there. its old school
if you are a blackjack player, don't play if the dealer is asian or an old guy named Sid. You will get raped.
if you win huge, do something stupid like rent a high end sports car on the strip and drive out to the Hoover Dam for a couple of hours. when's the last time you opened up a Ferrari in the desert?
after you check in, go say hi to a concierge in your hotel and ask for some advice about what to do, where to eat, etc. I've turned those conversation into comps, esp with a little gambling time in the home hotel too. make sure to get a comp card before you start playing.
tons of great shows. again, ask you concierge about discounts or comps.
if you are looking for a hooker or three, don't believe the card girls from the mexicans.
if you aren't looking for hookers but just want some eye candy, try to get into a pool party around town (best are Hard Rock, Rio & Palms). Its where all the strippers, hookers and hot tourists go to flaunt their stuff. If you have the scratch, big time it and get a cabana.
If you see a pack dudes hanging in front of the CVS or Walgreens, DON'T F with them.
Have fun, stay safe and don't loose all your cash the first day. It makes for a miserable trip.