We budget.
We budget $100 per day, and when that is gone, we buy no more. Many days we do not even spend that amount. This covers snacks, counter service, and junk (oops, I mean souvenirs).
Now, if we have an ADR, I also bring the estimated amount for that meal. We do not plan ADRs for every day. We rarely buy drinks at WDW, though the thought of having a drink at each EPCOT country is something we are considering as a lot of fun.
If we have anything else planned, like a tour or child's activity, I also bring that amount, too. That may be one per trip.
Have fun,