Thrifty Travel Mama had a while ago a great post on supermarket souvenirs from Scotland (and after that e.g. great posts about travelling in Switzerland. Awesome pictures and great tips for travelers). I meant to go to a supermarket with those “souvenir goggles” on but never got around to do it. But then Mel, a kiwi living in Finland, did something similar almost at the same time as Nina, a Finn living in Scotland, wrote about the food she misses from home. Both posts are good reads, and quite to compare.
As for the free but fun part: Food is always fun, no need for explanations, but food is heavily taxed here, so in shops it’s definitively not free. However, we still have a lot of relatively pure nature, so happy foraging!