We usually do our family vacations in a timeshare, so we typically prepare our own meals in the unit and eat out only a few times in the week. Saves money and is just so much more relaxing.
I'd really stick to sandwiches for lunch (I mean what would you have for lunch at home?) But also pick up yogurts, apples, bananas, and chips so it isn't JUST sandwiches.
A few cans of soup in different varieties and some lovely Italian bread can make dinner or lunch worth eating :)
Frozen, one-bag pasta meals or Asian meals are probably more expensive than you'd usually want to spend every night, but they are a LOT LESS than eating out and easy to cook without your whole kitchen.
Burgers are easy even without a grill as long as you have a stove/oven. You can use your sandwich fixings but its much more fun for kids.
Make your own pizzas with the premade crusts (that way they don't take up freezer space like frozen pizza does).
HTH
T.