Snacking in between meals is a good thing believe it or not.  It helps you to keep your metabolism levels up and going and it helps you to eat less at main meals.  Some would even say that you should actually have 5 full meals per day instead of the standard 2-3 depending on the person.  So realistically, 100-200 calories may be sufficient if you are not turning to 5 meals per day, but just really want snacks.  There are 100 calorie snack packs.  But if you use grapes, you can get 100 grams of grapes while taking in only 100 calories. 

In addition, since grapes are a negative calorie food, it takes more for your body to process them in calories than they actually add onto your body.  On the other hand, you can eat things like trail mix with or without m&m’s depending on how much fat you want as well as bananas, blueberries, walnuts, grapefruits, apples, whole wheat toast, etc.  There are various options and moreover various combinations that you can make for a healthy snack that will help you to lose weight rather than interfering with your efforts.  You just have to know how to do it right.