In my experience, which is not great, I am always keeping an eye open for potential emergency landing spots.

Your glide ratio should be about 1:1000. If you are cruising at 5,000ft (above the land), you have a 5 mile radius to set it down in. That allows for some roads although most roads in the mountainous terrain are useless for landing purposes. Open fields of weeds or farmland (landing in a plowed field is best for setting down parallel to the rows, not across the plowed rows).

Setting down on water requires a last minute abrupt flaring, so the plane actually stalls just above the water and drops onto the surface. An abrupt stopping will be felt as soon as the landing gear touches water (on a Cessna 182, keep the landing gear UP). As soon as the aircraft stops, release all restraing belts and open the doors to get out of the airframe. Most aircraft will sink up to the wings and will then float (IF the Wings are intact). The survivors can then either hang onto the wings or possibly climb on top of the wings until help arrives.
You will will find an excellent recitation of a water landing with wheels down in Ted Lawson's "Thirty Seconds Over Tokyo".

Setting down on tree tops are normally a "No Other Options Available Manuever" since the decelleration is very rapid while the tree limbs are ripping off your wings, breaking open your wing tanks normally and spreading that fuel around. If you mis-guessed, then you may also strike a tree trunk head on with the fuselage (Usually Game Over). Sometimes the tail wings will catch a branch and the aircraft is then suspended above the forest floor and the occupants will then wait for rescue. Sometime the fuselage clears the branches and then lands nose first on the forest floor (not usually a good outcome). Sometimes the aircraft clears the branches and lands on the forest floor lengthwise (best usual possible outcome) and the occupants can get out and tend to their injuries while awaiting rescue.

Making an approach to any Emergency Landing Sight requires paying strict attention to the location of all highline wires in the area. The VFR chart and your eyeballs are the solution. It does no good to have a perfect approach to a plowed field if you get hamstrung by that darn highline between you and your emergency landing location.