Good reply. It's a very difficult question to answer because everyone has such different tastes. We've gone with many different couples over the years, many of them going for the first time, and we ended up doing completely different things each time, as some people are into fancy restaurants, staying out till the sun comes up, etc, some are on more of a budget and want to mainly go to the free sightseeing activities, others want to soak up the sun at a cabana during the day, others love their shopping. So to give proper recommendations you really need a bit more info to help cater the trip to the specific person/couple.
I assume Bellagio was your favorite hotel/casino?