Berries
You can freeze whole blueberries, lingonberries, and cranberries without sugar. Softer berries like strawberries, raspberries and cloudberries require some sugar, about 0,5–1 dl per litre. This way their flavour, vitamin C and structure will stay better. Of course, you can also make slices, mush, jam, or juice!
Apples, plums, and cherries
You can slice or grate the apples. To avoid them getting dark, sprinkle some sugar and lemon juice on top. Plums and cherries will stay better in sugar juice with some additional ascorbic acid.
Mushrooms
Chantarelles, boletes, brittlegills and other mild mushrooms are heated in their own juices before freezing. Milk caps should be cooked in loose water, rinsed, and drained. Penny buns can be frozen raw. The best tool for a mushroom picker is a mushroom knife!
Vegetables
Almost all vegetables can be frozen. However, most of them should be pre-boiled, so their flavour and structure will stay better. Rhubarb, pepper, and leek don't need heating. Just chop and freeze them.
Herbs
Parsley, dill, and many other herbs can also be frozen as they are.