In both, the main female characters time travel to the future. Both dramas feature reincarnation. The main female characters' beloved/husband dies in the past, at the beginning of both dramas. The main male characters have reincarnations in the future. "The Story of Park's Marriage Contract" is a lighter drama, with more comedic moments, than "Thousand Years of Love". The latter also has more violence, love scenes and is more direct.

Both are about a group of friends who have different love stories. The plot also includes their professional adventures, but this is not an overwhelming part of these dramas. In both, the characters make important life decisions. Both dramas have a rather positive overtone, they are not depressing.
"Koi Nante, Honki de Yatte Do Suru no?" it's about some older characters than those in "Manatsu no Cinderella".

In "Twinkling Watermelon " the main male character accidentally travels in time and meets his parents.
In "Can You Deliver Time? 2002" the main female character accidentally travels through time and meets her parents.
Both dramas have love stories set in the past.