My favorite parenting tip ever (I think I got it from Dr. Sears) is the idea of a snack tray. We used to use an ice cube tray, or an egg carton, but now I just use a rectangular food storage container with smaller dip containers inside. Cut up fruits and vegetables into kid-size sticks or chunks, add a few healthy dipping choices, and store in the fridge. When kids are hungry for a snack, you can just pull out the tray. Older kids can help themselves (and should be helping cut up food for the tray, too).
We don't do snack try EVERY week, but probably 3 weeks out of every 4. I try to vary what's inside. I always include some sure hits, and then I add some fruits or veggies that are new to my kids, or that maybe aren't their favorites. I also add cheese and meats. On weeks we have a snack tray, that IS the kids' snack food. If they are hungry, they may help themselves to the snack tray, but nothing else. And it is not refilled. So if they eat all the grapes and are left on Friday with just Celery... so be it. I try to include dips that are both healthy and tasty to encourage eating the vegetables that maybe aren't as popular - hummus is a favorite at our house, sometimes I include peanut butter, or sour cream.
One dip that surprised me with its popularity was this Crunchy Veggie Dip. My 3 year old helps grate the carrots and stir, and my 8 year old is great at measuring out the ingredients.
Mix all of the following in a bowl: