Las Colinas is pretty straight forward business area. There's a lot of restaurants of various kinds up and down MacArthur from 114 up through/to 635. A lot of chain restaurants around the MA/635 area...and various others around MA/114.
Within the general/nearby area, you also have the Grapevine Mills area - outlet mall with movie theater and more chain restaurants. There's also the Gaylord Texan Resort down the street - The Glass Cactus is their nightclub/bar that overlooks the lake. Some great talent, especially cougars, but expect to get bent over for drinks.
If you're willing to venture out further - restaurant row is located in Addison off 635 & Dallas North Tollway. (You'll need to venture up DNT to Beltline) Beltline has restaurants (chain, unique) of every kind, so depends on your fancy. Plenty of bars - Dukes, (popular hangout, sometimes filled with bikers, great patio) Black Finn, (chicks) Sherlocks, (more chicks) Logans, etc. Addison is smoking though, FYI.
Further up the Tollway is the Shops at Legacy in Plano. Think of it as more refined and fancier. I tend to hang out at the Shops more than anywhere else - smoke free, plenty of good places to eat. Recently opened up The Gingerman there. (Great place for brews)
The "happening" spot in Dallas tends to be Uptown, where the $30 Thousand Dollar Millionaires tend to all live/hang out. The latest restaurants and bars of all kinds - it spans quite a large portion of the area around downtown.
If you're looking for downtown action but less pretentious/more lowkey, Deep Ellum is the place to go. It's slowly being revitalized - a few bars here and there...very artsy.