Easy way to get ontop of things.
Always keep a plank on you.
The plank can be carried ontop of the car if you dont drive like a maniac, or on a trailer with a box holding it down.
When at the destination, Simply rotate the plank to upright position and let it fall onto the wall, making a ramp you can do the basic half life crouch jump, to get ontop of the car and onto the ramp.
Good stuff are hidden on top of buildings, much worth.