Plan for the next school year. Read the books they are teaching again, and again. Develop more curricula. Take classes to develop professionally (which is actually required of teachers as part of salary increases and evaluations).
Also, teachers do have hobbies that they like to participate in just like any other professional. The exception is that unlike other professionals, teachers rarely have time during the ten month school year to engage in these hobbies since their days are spent in the classroom and their evening are spent grading papers and planning. So, the summertime is the time to do those things, such as tennis, or writing, or traveling.