You are not alone! A lot of people have posted in the past about the same problem, including me!
The solution, for me, is to PLAN my weekend days out. I am forced to plan my weekdays because I bring everything I eat. Do the same thing on the weekend! Make your snacks, lunch, etc in the morning and put in the fridge or on a special counter. Then you will know that is what you have for the day.
As for the water, I got a water bottle and bring it EVERYWHERE with me. When I go from the living room to the computer room, it goes too. I manage to get in all my water this way, weekday or weekend!