Nothing speaks of love more than Paris. So how about taking her to dinner at the Eiffel Tower Restaurant at the Paris Casino which overlooks the city? After a romantic meal, take the elevator to the top of the tower, and while viewing the city lights, propose!
Do you want it to be a public proposal or something more private?
If you want something public, the Eiffel Tower was a good idea. Or you could go propose while watching the "Fountain Show" (it may have a real name--I just don't know what it is) in front of the Bellagio.
Or you could do something more cheesey (but fun) like split up in the casino to play the slots, then come and find her and tell her you just won the jackpot and she'll ask what you won and you get down on one knee in the middle of the casino, break out the ring and propose. (I think that is allowed--they have so many darn rules in casinos one can never be certain!
If you wanted to do something more private you could propose in the room before an outting to the casinos, telling her you want to give her something for good luck. Then have a bottle of champagne waiting for a quick celebration.
You could go visit the desert and do it there! The ideas are enless. Let us know what you decide and how it goes.