Muffins. They are the perfect handheld treat. Make them ahead of time and keep them on hand to offer any hungry troops who need a snack. I like to make muffins out of unusual flavorings: both savory and sweet. My savory muffins are a good breakfast on the go, while these blueberry muffins are great as a substitution for toast on the side of a nice breakfast.
My kids love that they can take one out of the freezer and take it with them to the park or the local pool. Then they have a handy snack (free), that will actually fill them up more than a bag of chips or other junk food. To be honest, the teens grab at least two. They are eternally hungry after all. My oldest son says he needs one to tide him over, while he opens the second one.
Remember that you can substitute just about any fruit when it becomes available at the farmers market! Try raspberries, strawberries, peaches, cherries - the possibilities are endless. Get out there and bake!