If you liked the K-On! manga, the Anime-Planet community thinks you'd like:
The after-school club is a frequent theme in manga. In both the cases of K-On! and Manabiya, the club forms the setting for humorous, oddball activities that often have very little to do with the club's stated purpose.
Life is a fun adventure for sixteen-year-old Nobue Ito, her ‘average' twelve-year-old sister Chika, the mischievous Miu Matsuoka, timid Matsuri Sakuragi and ‘English' Ana Coppola. Together, these five friends assist each other in the most adorable ways possible. Whether they're teaching someone how to swim, helping with summer assignments, preparing a surprise birthday present or shopping for the cutest clothes, these girls always manage to have a good time no matter what the season or occasion.
Both K-On! and Strawberry Marshmallow share a similar style of comedy. While Strawberry Marshmallow exhibits more of a slice of life quality, if you enjoyed one you are likely to enjoy the other.