There should be no taxes or any (involuntary) additional charges. Of course, if you raid the min-bar or have dinner at the hotel, etc., they will charge you for that.

Breakfast probably not included but, if you're a Diamond, you should get access to the lounge (if any) which will supply lots of food/munchies. A lot depends on the individual property.

I've only made a few award stays (as a Diamond), but in every case we were upgraded, well fed (lounge and/or restaurant), well liquored up, and never had an additional penny on the bill.