Different people find that different types of snacks work for them. Some people want a little protein in their snacks, some a little fat, some a little fiber - some find that unless they have all 3 in the snack, they'll be back for more in no time.
I always leave room for planned snacks because I enjoy eating, so I find it a pleasant break in my day.
I tend to like the protein/fat/fiber triple-whammy, but I don't always need all three in one snack. Here are some of my favorite choices:
* Apple + a little bit of a low-calorie cheese (my favorite choice is the middle eastern cheese Nabulsi, a brined fresh cheese at 80 calories/ounce)
* a boiled egg, sometimes with a handful of fresh baby spinach leaves
* Greek yogurt (strained yogurt very high in protein) - I tend to save this for dessert a couple nights a week, but it could make a good snack too.
* Banana; clementines (or mandarin oranges); kiwi fruit
* Popcorn. I make this two different ways. At work, I keep unpopped kernels and brown paper bags in my office. 2-3 of tablespoons of popcorn in the bag, fold over the top, and microwave for 2 minutes = instant no-fat-added homemade microwave popcorn, a nice little serving at about 100-120 calories.
At home on weekends, if I have a few more calories to spare, I actually pop corn the old fashioned way on the stove with a teaspoon or two of oil. It's absolutely delicious and only adds 40-80 calories to the popcorn. (I actually tend to use a tablespoon of oil to 1/4 c. of kernels, and split the results with my partner, for about a 200 calorie snack for me).