What do teaching faculty do during semester breaks?
In the United States, the "9 month salary" for a faculty member is typically paid out over 12 months.  For those faculty who engage in research or other pursuits during the summer months, the "9 month" designation gives the rate at which their salary can be enhanced through external funding.

So, what do "full time teaching" faculty do over the summer?  Colleagues of mine in such positions generally still look at this time as an opportunity to focus on their professional interests, whether that is doing research or some other field-appropriate equivalent such as writing books, directing theatrical performances, etc.

Teaching responsibilities may also extend through the summer (and may be additionally compensated or not, depending on circumstances). Examples include teaching summer courses, preparing new course material, and supporting student groups' activities (summer is a common time for technical competitions, for example).

And some, in some circumstances, really do just take the summer slowly. In my experience, though, that tends to be the exception rather than the rule, with so many other opportunities and responsibilities at hand.