Canadian North B 737-200ADV "Combi" at Yellowknife Airport NWT, with a gravel kit that allows it to operate on gravel runways, the first few rows can be removed to make space for large cargo, while the rear half can seat several rows of passengers.
The nose gear has a plate attached ("ski looking thing"). This is to deflect any gravel etc that the nose gear kicks up, protecting both the underside of the fuselage and the engines.
The nose gear wheel well is modified to make room for the deflector to retract into.
The engines have vortex dissipators installed. These are tubes under the engine intake that direct bleed air to prevent vortices that would normally suck gravel and what not into the engine.
I've been told that the rotating beacon on the bottom of the aircraft is retractable to prevent damage to it, the bottom of the inboard flaps has an extra protective layer stuck on, and in the main wheel wells there are cage-like things installed to protect the maze of flight controls and hydraulic lines and components in that area.
Canadian Aviation Blog