If your not renting a car, you might want to get limo service to and from the airport. It costs a little more (not much) than a cab and will pay off big with the girl friend.
Presidential limo has a round trip deal with champagne and a rose for her.
Sign up for email specials at all the hotels you would stay at. I get emails everyday with deals. Venetian is nice so you can't go wrong there.
Wynn is close to the Venetian and worth going over for a drink and a looksie. Nice place.
If she wants to shop take her to Ceasers forum shops and you can sit at Casa fuente and have a cigar while she shops.
There are 4 different circ shows. I've seen them all but the naked one. Bettles/love is the best imo.
Use that show ticket discount above if you want to save some bucks and are flexible on what show you are going to see.
Hope you have a great time.