Yes, we have experienced summer slowdowns at our space. Our county/community is the oldest (median age wise) in the Bay Area, so we have lots of people who have kids and grandkids, kids home from college etc.
So summertime frequently brings other priorities.
Besides letting people know that there's a natural slowdown every summer, as long as you have the space/facilities for it, I would do some things like and not limited to the following:
lunch & learns, wine tastings, yoga classes, live music night/jam, hackathons, maybe a STEM night or Maker Faire, startup pitch and feedback sessions, cannabis talks if you're in a state where it's being legalized and what the opportunities/implications might be, etc etc.
Look at other coworking space websites to see what they're doing activity-wise.