Vast majority of Hilton properties in the US offering free breakfast buffets to Diamonds will have you sign a check for $0 with a tip line. I hate the whole concept of tipping, but I invariably write in an amount for ~15% after tax.

I was just at a DT in Cleveland, and I had just finished up my breakfast (free as a Diamond), signed the receipt/bill with tip added, and headed back to my room. Then the phone rings, and it's the restaurant staff telling me I need to head back down again because I didn't do the "bill." I told her I did, then she puts me on hold (I was in a hurry, too), then she admits to realizing she had the signed check in her pocket. She's so absent-minded as to forget she actually had it on her, so shameless as to bother to call me for that $3 tip, and then so out of touch as to suggest that I should go to them instead of them come to me. Even then, I'm expected to tip.