Lot's of great recommendations. I personally would second Kimi Wa Petto, Witches' Romance, What's up Fox, Last Cinderella, Dal Ja's Spring and Secret Love Affair, all of which I really enjoyed.
There are two historical ones with an age difference but as often is the case with historical dramas, the ending is not necessarily a happy one. They are Ooku and Painter of the Wind. I found both of them absolutely beautiful.
Also worth mentioning are I think are I need Romance 3 and The Lover. Both are a bit more 'real' in comparison to your average Korean drama and make a refreshing difference,
