Recommendations really depend on several things:
What you would like to do/see while in Las Vegas
What is your idea of romantic - in Las Vegas it can be ridiculously over-indulgent or tasteful 5* pampering.
There is something for everyone there as long as you know where you fit on the spectrum, you'll be fine. But it's not cheap so be prepared to spend some money.
Here are some ideas of things to do:
Dinner at the top of the Stratosphere (revolving restaurant)
Helicopter ride to Grand Canyon
Gondola ride on the Venetian canals
Cirque du Soleil shows - ZUMANITY is the sexiest but all are awesome.
Ride the High Roller - the world's highest ferris wheel
Lots of great food to enjoy