Boss and Me (cdrama): The story of ShanShan, a happy-go-lucky girl that donates blood to the big boss's sister because she has the same special bloodtype. Very upbeat story for the most part, a lot of episodes but almost all of them are exclusively about the couple (no boring politic sidestory for instance) childish lead (gets jealous but pretends to be cold etc) They didn't know each other beforehand though.
A Love So Beautiful (cdrama): She falls in love first and is very open about it, while he pretends not to care but then does anything he can to take care of her. Very cute and upbeat. They fast forward near the end, after a few years have passed and they're grown ups, he hasnt forgotten her at all but she wants nothing to do with him. Fits your criteria I think, but most of the drama is in the before stage.
My ID is Gangnam Beauty (kdrama): Fits all your criteria! New each other beforehand, childish ML, she's stand off-ish at first.
Something About 1 Percent (kdrama): Contract dating. Very fluffy and romcom-y.
I also agree with the dramas mentioned above, What's Wrong With Secretary Kim especially.