Pack ahead! Think portable, non-perishable, and healthy. Dried apricots, granola bars, apples, bananas, oranges, nuts, etc., etc. You can buy in bulk at a wholesale club or clip coupons and stock up when snacks are on sale at your grocery store. Carry reusable water bottles as well and you can refill them pretty much anywhere! You'll notice a savings right away when you stop buying snacks on the fly.
What kind of snacks do you bring along with you? Do you have any tips for saving money on snacks?