We spend Saturdays outside mostly

We find ourselves eating out for lunch and breakfast and/or dinner too. A fair amount of the day is spent at the park, so we do have picnic tables, etc. We’ve been trying to get food at a grocery store and eat it for breakfast and lunch but by dinner time, we’re starving and a cheese sandwich just isn’t going to “cut it”. We do drink water all day long and refill our water bottles for free.

Any cheap, (mostly) healthy suggestions on how to avoid eating out when you’re out all day long?