Are there restrictions? We have one nut-free room that always makes it more challenging and they try to avoid dairy because so many kids are lactose intolerant. There is also a stated guide of "healthy", though that is left up to each parent to interpret. We also have "routine" snacks and then twice a year (birthday and "start of the week") the kids get to bring a special snack. For special snack days I usually bake something because I love to bake. For routine snacks I try to send a fruit and something else.
Popular fruits include: bananas, apples, grapes, tangerines, berries
Other options that are popular: pretzels, cheese crackers, popcorn, granola/protein bars, cookies (I generally make oatmeal ones), sugar pita chips, mini bagels, peanut butter/sun butter (no nut alternative) and apple slices, mini carrots and ranch dip, cheese (especially baby bels),
Some special things that I've done that went over really well: mini apple or pumpkin pies, pumpkin empanadas, fruit smoothies (have to be made and taken quickly)