L.H.
We travel a lot. One trick I use all the time is to freeze bottled water a day or two before traveling. That way the cooler is packed with frozen water bottles that serve a dual purpose. You also don't have to worry about things getting down in the water once ice starts to melt. May not work in what you're planning but I LOVE doing it that way. Lots less mess and if they are frozen solid it takes several hours before they start to thaw.
I love fresh fruits, cheese and veggies for a picnic. I always pack peanut butter and almond butter to spread on bread or fruits. Personally I LOVE the crusty french breads where you can just tear a piece off to share. Different types of crackers, (gold fish or something similar for the little ones). Trail mix with nuts and dried fruit is yummy. Pimento cheese is another spread I like but tend to only fix what I think we'll eat because I will throw it away if it's been out in the heat due to the mayo content. I think everything tastes better when eaten in the fresh air and sunshine!
Enjoy!
L.