[MicroBob]

Fountains at Bellagio. GF really liked it.

Dinner at the top of the Stratosphere.
Dinner at other LV nice restaurants too like Nobu or Delmonico's or wherever....but I think the top of Stratosphere has a more romantic feel to it even if the food isn't as spectacular.
GF really liked it. Slowly spins-around as you are dining so you end up seeing the whole city.

Even just going up top to the bar could work alright I suspect if you want to do dinner somewhere else.

Night clubs or ElD's preferred lounges can both work in different ways I think and depending on your mood.

I think there's a restaurant somewhere up in the eiffel tower at the Paris where you can get dinner or just coffee and also have a good view of the Bellagio fountains. Didn't go there myself but that might be worthwhile.

Fontana Lounge has a good view of the fountains too, doesn't it?

The cheesy gondola ride in the Venetian. We didn't get to this because the lines were kind of longish when we were there. But GF wants me to take her back to LV partly just so we can do this.

We also didn't hit any of the Cirque shows or anything. But I've told her that I've read on here that supposedly they really are worthwhile so we'll probably go to O or Ka or something else upon our return (I really don't think either one of us would be interested in sitting through a really long stretch of Beatles music).

didn't get up to Fremont Street. No idea if their little light-show wha-cha-ma-thing could also qualify as 'romantic'.

ElD's recommendation of room with jacuzzi tub is a good one although would certainly be more expensive in LV I suspect.
We have done this with the mini-jacuzzi thing at the Sheraton in Tunica.
Not bad after a few hours of gambling and drinking. Light a couple candles and it feels like romance-city.