Paragon/Siam Discovery is a good suggestion. MBK is just a skytrain stop from Paragon/Discovery though thats a ton of mall stuff to take in.
Platinum (cheap clothing) or Panthip (electronics/etc) if you want shopping. Central World is right around the corner from there and Baiyoke Tower across the street.
If hungry get the buffet at Baiyoke Tower. Highest point in Bangkok and a great city view (restaurant on the 78th? floor). Around $20 and pretty good food.
Didn't read it earlier, what time are you there ... day or night?