congrats on booking your surgery. You will love it! I too was not even hungry after surgery, Had to force myself to drink Water and the liquid part. I found it toughest once I hit the puree stage. Just be careful as this is the healing stage.
Diet Coke? I was an addict, had my first one for Breakfast, and didn't quit all day. I quit cold turkey after my surgery (I did sneak some in the pre-op diet). The carbonation is not good for the band. However, at the bash, it was amazing how many people were drinking pop. I think they try to emphasize not to do it too often. Actually when I had my first one post-op (it was at least 5 months post) it didn't taste the same, and I could not even drink the whole pop.
Good luck with your journey...