Fruit is an obvious option when you mention "healthy snacking" but be careful which items you choose and how you package them to handle long periods of heat in the summer. Apples are better whole because when cut they will start browning, and avoid berries or banana because they are easily squished in your bag. My favorites are: whole apples, raisins, mandarin oranges, and cut up grapes
If you have a toddler that fights eating vegetables, sneaking them into a healthy snack is the perfect place to make sure they get their greens. We recently found Gerber's Organic Veggie Crips and Oliver LOVES the Farm Greens flavor (they also have an orange harvest flavor that I haven't found in our store yet). They are made with real organic vegetables, whole grains, way less sodium that other veggie straws/chips, and come in convenient individual packages for easy snacking on the go.
p.s. There is currently an Ibotta $1 coupon for these that you can find on their website or app.
Fresh vegetables are tricky to pack and keep good in a diaper bag. I definitely prefer the dried Gerber Veggie Crips I mentioned above or fruit. However, fresh carrots do well as long as your child is ok eating them without ranch (the mayo in ranch will spoil in the heat). I don't recommend celery or broccoli as I've always found it gets limp and gross if not eaten soon after refrigerating.
Some other favorites are: Cranberries, trail mix, low sugar fruit snacks (they do exists!), cheerios, baked gold fish, and fruit bars (keep them in a pocket so they don't get smashed)