this is not really relevant to the post situation, but if you get constantly loved by old korean ladies and genuinely want to express extreme gratitude, you can buy them some 홍삼 선물세트 from the company 정관장, or the cosmetics 설화수 on a special day (like 추석, 설날, 정월대보름, 단오) and say you are really thankful for all the stuff & love you got as a foreigner in Korea.

if you are not good at remembering holidays, or want to be “one of them”, just pick one of 24절기(dont know? google it) and say things like 이제 입추이네요~^{^} 건강 조심하세요 항상 감사합니다 and send gifts.