Well, it all depends on whether you're being comped or paying out of pocket. You also didn't specify what games you like to play.
Best place to gamble - wherever you feel lucky. If "eye candy" is your thing, the D and Golden Gate have very good party pits at night.
Best seafood buffet - not much downtown specifically for seafood. You may need to go up to the strip, I highly recommend the seafood buffet at the Rio. It's expensive $50+ but well worth it. Check out my review in the restaurant section.
Best Steakhouse - I have eaten at Andiamo's at the D about 8 times and never had a bad meal. I have also heard that Hugo's Cellar at QQQQ is excellent but more of a "date place".
Unique LV gaming experience - Silver Strikes at the Four Queens - a note (if there aren't and red caps or blue caps in the strike turrets, just play to get one clear cap, since they are only silver plated)