Depending on what aspects of that stay are important, I would suggest the following:

BIGGER/ROMANCE/MULTIPLE NIGHTS STAY

Four Seasons Resort and Club Las Colinas

Omni Mandalay Hotel

Gaylord Texan (although it's really big)

Mansion at Turtle Creek

Crescent Hotel

COZIER/ROMANCE/B&B TYPE

Etta's Place - Sundance Square (Fort Worth)

Hotel St Germain

CONVENIENT TO AIRPORT/ONE NIGHT STAY

Four Seasons

Omni Mandalay

Marriott DFW North

Sheraton Grand DFW North

Harvey Hotel DFW North

Embassy Suites DFW South

My wife and I stayed at the Adolphus in downtown Dallas for our wedding night, before we went on our honeymoon. Ask for the honeymoon suite. There are few better rooms in Dallas. Large room, sitting area, balcony, etc. This is a first class hotel.

Hotel Zaza is a new, hip hotel in a great part of Dallas. They have several themed suites to choose from and a great bar. HotelZaza.com

The Mansion on Turtle Creek or The Crescent Court Hotel are the nicest in Dallas.

Rosewood.com

The Melrose Hotel is also very nice for usually less.

hotel zaza is a chic boutique hotel in uptown.

the magnolia downtown is also nice. it's the hotel that has the red pegasus on top.

both have a real modern feel.

the oher suggestions that were given were good ones, some were really spread out...the gaylord is in grapevine, and some of the others she mentioned were in ft. worth and then in n. dallas.

i think you should narrow down an area of town you want to stay in, then decide.