Could a person damage a jet airliner - from the outside - with their bare hands?
Yes, a few possibilities...


Brake hydraulic and electrical sensor lines located on some landing gear.
Putting foreign objects under or between the rotors of brake pads
Covering pitot system sensor holes
Pressing or hitting the pitot tube (on most airliners you will need a ladder)
Letting air or hydraulic fluid out - under inflating tires or cylinders
Bending or mis-adjusting bellcranks/control arms or changing the linkage geometry.
Damaging fuel vents


I am not sure of what your definition of "exposed" is but most airliners allow the E&amp;E (electrical &amp; equipment) bay usually located above or behind the nose gear and gear doors to be opened and closed from external control panels for ground crews. This allows access to most the computers, batteries and thousands of wire bundles. It may also give access to lines, cables, pumps, switches, and sensors. 

Opening the gear doors of some airlines exposes the air-packs, generators, converters and aux turbine with it's associated mechanisms. Maintenance and inspection panels, engine cowls and fuel distribution panels expose or control critical systems.