Water only. Food wise, I'm a lot more lenient than I thought I'd be...easier to vacuum some goldfish off the floor than listen to whining IMO! The only thing I'm strict about is anything that can melt and become a nightmare to clean if dropped on the floor.
We had chocolate milk spilled too many times so that is the only thing I really try to keep him from drinking unless it is in its own cup with a lid and straw.
No rules really except I don't give them a drink that's not in a sippy cup. We don't let them have stuff that is undoubtedly going to be a big mess like a chocolate bar or freeze pop or something obvious like that.