Have a good dinner -> 맛있게 드세요 or 맛있게 잡수세요 or 많이 드세요. 맛있게 is literally "savoring it" or "fully tasting it", and 많이 is "in good quantity", "heartily", "abundantly". 드세요(들다) and 잡수세요(잡수다) are both "eat" in polite and honorific form. "itadakimasu" is something you say when you start eating, sort of a "thank you", and is literally "we receive/take it". For that, Koreans usually say "잘 먹겠습니다", which loosely means "we'll eat with gratitude". 잘 is a common adverb with meanings like well, in good manner, nicely, skillfully, and so on.