There is nothing significant in the areas you can not reach. The only readily accessible places on the mountains are the imp castle and the warrior's guild.
To clear as much fog-of-war as possible, try a combination of;
- Max Elven Sight (much more effective) and Ranger Sight (if you have the skill points to spare and really want to maximize your sight range).
- Keep any equipment you find with sight bonuses, to use when exploring.
- Toss a teleporter stone onto/into inaccessible regions and move around by repeatedly tossing one stone and teleporting to it (you are not suppose to be up on most mountains, across the river in the south east corner of the dark forest, etc, and will not be able to walk there).
- Cast Wizard's Sight twice in the same location (cast it a second time before the fog-of-war fills in again from the first, and it will remain clear). Hold the shift key down to move the screen focus away from your character and towards the area you want to clear.