A veggie soup? You can prep in advance, even freeze in portions. Edit to add: my fav meatless snacks are lentil quesadilla, sautéed cabbage and leftover rice/grain, zucchini fritters and savoury oatmeal.
I like to make Everything Bagel peppers. Stuff mini peppers (I buy the variety pack from Target) with a mixture of cream cheese and Everything Bagel seasoning. Easy, filling snack thanks to the combination of fat and crunch.