Le Royal Meridien looks like it's having a pretty sweet deal right now: 410 AED for the night.
I know absolutely nothing about Abu Dhabi, but I think I'd prefer downtown to Yas Island. I think, as has been suggested, we'll have dinner, grab some drinks (clubbing isn't our thing), take a stroll along the Corniche, and dip our toes in the Gulf.
csdf, since you're an Abu Dhabi local, could you share some of your favorate moderately-priced restaurants and/or bars (either itt or via PM)?