If I am just driving locally (Within 100-150 miles of home), I tuck a short list of towing services in my wallet, charge my cell phone and hit the road.
Years of living with a 356 and a 912 as daily drivers taught me that on long trips I should carry two fan belts, a complete generator pulley with spacers, a set of plugs, a couple of quarts of oil, a set of wrenches (socket & open end), screwdrivers, pliers, and wrench suitable for removing the nut on the generator pulley.
Oh, and then always understand that no matter what you carry, something you aren't prepared for will break.