Mob Museum (downtown, two blocks North of Fremont Street). I highly recommend adding an optional item as well. Distillery tour/tasting and Crime Lab are two I've enjoyed.
If you drink, go to I Love Sugar's upstairs bar. Between the two couples, get at least two giant "Martini's", one small drink, the boiled up Gummy Bears and the Lik-M-Aid.
Enjoy at least one nice dinner on the Strip. Smith & Wollensky if you've never been to one. Maybe Lawry's the Prime Rib, which is my favorite, but is three blocks East of The Strip.
I'm guessing you've been to the top of The Strat (FKA The Stratosphere). If not, go there. Maybe try one to three of the thrill rides as well. Dinner up top is excellent with excellent service as well. Pricey but it's Vegas.