Peanut butter. You can get a kilogram of the stuff for like £3.99 from Lidl and Aldi. Add a generous layer to your your rice cakes, it tastes really good with marmite too, also excellent with banana, apple, jam, honey etc. as well as sriracha. Dollop some on a yoghurt bowl with some fruit and some seeds. Mix in with a stir fry. Eat it straight from the tub by the spoonful.
Add butter to things where you can. Buttered rice cakes are quite tasty, especially if you also put extra toppings on. Butter on potatoes, pasta (you can get
"protein pasta"), mash (including vegetable mash), veg, or use olive oil for salad dressings etc.. Use a generous amount of butter/oil when cooking – almost everything tastes better when it's been nicely cooked in butter
Cream cheese, cottage cheese, normal cheese, houmous, avocado… use as dips, spreads, sauces (e.g. in pasta), or eaten straight as they are. Get the full fat stuff.
Nuts and seeds: nice to snack on, easy to pack and transport, and can come in lots of different flavours.
A few (peeled) hard boiled eggs. Tin of tuna. Depends what you class as a "snack"!
Smoothies might also be an option? Just chuck a load of stuff in a blender and then drink away, easy calories in bish bash bosh. Banana, peanut butter, milk, yoghurt, protein powder, seeds, other fruit/veg, whatever you fancy and have on hand.
You can buy protein bars, lots of different types are available nowadays, can be quite pricey though. Eat Natural, Kind, Nature Valley do some fairly nice nut-based bars; I haven't tried many other brands so can't really give many more recommendations. Most packaged foods have some sort of nutrition label on; don't get too caught up in the numbers but they can give you a good idea of protein and carbohydrate content as well as calories.
Basically just eat as much as you can get your hands on, put on a few kilos, and hopefully by then you'll work out some decent eating/snacking routine (set alarms if you need to) that is sustainable for you.