Entirely dependent on what kid likes, same as kids attending co-ed schools. Community theater, local choir or orchestra or band (school of rock, etc); local/traveling sports teams; art classes or other kinds of enrichment activities (sailing, swimming, cooking, coding, martial arts). Summer camps for sure.
Just know that as they get into HS they will have much less time for non-school activities. Things like music or theater would likely be coordinated with a brother school, though.