Like @ricky says above, Reply 1997 is really great, one of my favorites :)
I Need Romance 3 is one of the only age gap dramas I can think of. The main lead (younger) is so, so, so in love, but the female lead takes some time to come to terms with it. I've rarely seen a drama where the male lead was so openly in love though.
I Hear Your Voice is said to be a mix of genres, which it definitely is, but most essentially it's romance. It's another noona (=older woman) romance, and I guess it must be the case with most younger romantic heroes being more pure and direct with their love, because again, this drama's male lead is head over heels in love with the more jaded, cynical female lead. However, in this drama she acknowledges her love for him earlier, which leaves more time for actual couple moments. A lot of people loved this drama, and so did I :)
Here are a few non age-gap romantic dramas with chemistry I can think of:
I saw you haven't watched My Girlfriend is a Gumiho, which is a classic. It's not my favorite out there (simply because I'm not a Lee Seung Gi fan) but it's still a really good drama. The female lead, Gu Mi Ho, is different from any other drama character you'll ever see, and since she's so honest and authentic, she really pushes the romance far, without the usual hesitation/denial dilemmas.
Boss & Me has a lot, lot, lot of chemistry: it's pretty light (as in there's no MAJOR, dramatic event), quite innocent, and certainly romantic. I think it's the drama that focuses the most on plain, simple love I've ever seen, hehe.
Que Sera Era has very powerful chemistry too, but in a more raw, almost sexual way - if you want to avoid dramas where the characters are mean to each other half the time, then you might avoid it. I loved its intensity and originality, though.
Good luck!
