Tips of healthy food

Eggs
Pretty filling (imo anyway), and healthy. Decent protein for not so many claories too. 3 large eggs, scrambled can be like 210 calories. butter and milk to make them creamy/fluffy too. Replace normal milk with like protein milk or something, if you wanted those extra for Gs of protein too. With those extras, even if you were 'generous' using them, it'd still be under 300 calories. (although that'd be more breakfast sized than snacksized for me. so 1-2 eggs with same amount of milk/butter would be more snacksized)
Keep in mind. Cholesterol does not equate to ldl in the body, so daily eggs are no problem, if your general diet and exercise is healthy.
scrambled may not even be the most filling, i'm not sure, they're just my favourite. eggs anyway are good. But maybe even hardboiled might be more filling than other ways.

Slice of Toast and Peanut Butter
Again imo its filling. And that can be under 100 calories. So you could have 2 separate slices, with peanut butter spread on both of them, for under 200 calories. And if you use like a wholegrain/wholewheat bread with seeds on it (the one I buy is like a 9 grain, multi seed bread @ 86 calories per slice. Although the slices are a bit smaller than normal), even the bread would be high enough in protein and fiber which are filling. Get a good peanut butter made from 100% nuts only, then its all healthy fats and protein (without worry about extra sugars or stuff added, if you don't care about that stuff, then np lol).

Fruits
Again, depends on how much you care about sugar, or how much your daily limit already is.
1 Banana + 1 apple, could be under 200 calories. (banana being around 110, apple about 85, depending on types/sizes).
Or personally, I love berries. Expensive as they are here. I still go through 1-2 boxes / day.
125g (the standard size pack/box they come in) of blueberries is 73 calories I think, or something similar. Raspberries, have even less calories, more fiber, and less sugar than blueberries. Blackberries are good too.
Or something i've never tried / can't find to buy here are Elderberries. They have plenty of fiber, but 0 sugar. Although because of that, not sure if they're edible as something nice, or only used for cooking, never looked into them.

Cold Meat
Y'know those packs of ham, chicken, turkey, etc slices that you buy to put in your sandwhiches? Just a pack of those on their own. The majority of calories from a standard sandwhich come from the bread (usually 160cals from 2 slices of normal bread) or cheese (about 70cals for a slice/single). Again, buying packs of them are expensive enough, but I mean if you enjoy the meat and have some money, its a good filling snack, high in protein, low in calories.
Also, i'm talking about the low-no processed stuff. Not the kiddie kinda packs where the slices are papery thin, dozens are squished into the pack and barely even taste like meat. But ones that are as close to real solid sliced meat.
I buy 140g packs of chicken or turkey slices (3-4 thick slices that you can see were carved from real breasts), for €2 each. And I think its about 130 calories for the entire pack, 28-32g of that being protein - which is filling.

Nuts
They're nice for snacking. Completely healthy. Good fats and protein. Just that they're calorie dense. So it wouldn't take much to give you 100 or even 200 calories. I love a bunch of different nuts, but rarely have them because what I would consider filling for them, would be more calories than i'd have in even a dinner

Cereals
A bowl of cereal or porridge or something. Porridge, if you use water (instead of milk), could be under 200 calories (or just buying those quick pots of porridge that you fill a little with water and microwave for 60 seconds, are generally, under 200 calories)

Leftovers
snacks don't have to be the typical or usual things. Most vegetables are low enough in calories. So if you enjoy them, then make extra for dinner, and keep some left over for snacking on the next day. You could even have "mini dinners" as your snacks, by using the leftovers in small enough quantities. like 100g mashed turnip, 100g mushy peas, 100g roast beef or chicken or turkey or whatever, and 10-20ml gravy. I'm sure that'd be around 200-250 calories.
So if you made a big roast dinner one day. And had left overs. Then the next day as a snack, you'd be able to have a mini version of that dinner again lol.