I echo the 2 suggestions here. We greatly enjoyed the Paris buffet for the breakfast/lunch turnover, for something like $6 after the $10 off coupon. Weren't overly impressed w/Mirage, but it was a solid meal and good deal w/the 2 for 1.
Tried Planet Hollywood for a weekend brunch and was very disappointed. Depending on what day of the week you'll be there, we really liked Garduno's Margarita Brunch at Palms, though that may not be your best use of the 50% off coupon. Rio was ok for us--definitely lots of choices. We've never been disappointed at either Green Valley Ranch or Red Rock, though it sounds like some have found 1 or both wanting more recently.
Hope that's helpful!