When we did our build, we would bring out cold drinks (sodas), sometimes sandwiches if they were there long enough, and donuts. It was our way of saying thank you to the subs that were there longer and doing some of the "bigger" jobs. (i.e. excavation, decking/pavers, masonry work). For something like this, I do not tip in cash, as they are already getting paid what they agreed to for the work they are doing.