I brought carrot cake muffins decorated with just a little frosting (made smiley faces), mango juice from Trader Joe's and small red gala apples (the really small ones). The kids LOVED it ALL!! Keep it simple! Half the time, you have kids that can't have certain things due to allergies, so I always try to keep it simple and have something that everyone can enjoy. Also, I showed up during their afternoon snack time so rather than snack on what the school usually gives them, they got what I brought.
Another fun idea might be to read them a book then supply a snack that pertains to the book. For example, If You Give a Mouse a Cookie - you could provide those little Organic chocolate milks from Costco and chocolate chip cookies.
Also, my son's birthday was recently and we spend a lot of time at his sister's school so I brought treats for his birthday too. I brought watermelon. Who would have thought the kids would have been so excited! I brought enough for the teachers in all the classes but these kids ate it all and they still talk about it.