If you're looking for manga similar to K-On!, you might like these titles. All recommendations are made by Anime-Planet users like you!
Kodaka is a delinquent. At least, that’s what his fellow classmates seem to think. Thanks to his naturally-blonde hair and an unfortunate series of misunderstandings, the boy can’t seem to shake his false reputation and hasn’t made a single friend. That is, until one day he overhears the normally-aloof Yozora enthusiastically chatting with an imaginary friend, and discovers that she’s just as lonely as he is! In an attempt to finally meet people the duo decides to form the “Neighbors Club”, and are soon joined by a number of other fellow misfits such as Sena, the busty, popular daughter of the school president who can’t seem to connect with others. Together, the gang learns about the joys of friendship through many misadventures such as playing RPGs, visiting the swimming pool or even hunting down delicious takoyaki!
Both Boku wa Tomodachi ga Sukunai and K-On are manga based around a really slack highschool club. They both tend to get very off track with club activities so members mostly hangout or end up doing random activities. Yozora and Mio are also very similar, mostly in looks, but in shyness as well. Fans of one would enjoy the other.