Catch a free kyogen (traditional comic theatre) performance, make your own handheld fan and learn how to wear a yukata at Nogi Shrine's annual summer festival, which is usually a lot more laidback than most of the city's other natsu matsuri. There's also a brief selection of food and drink stalls and a special version of the shrine's popular antique flea market.
Note that the kyogen shows (afternoon and evening performances) have limited seating, so best arrive early if you want to catch one.