Fantastic Terrain is where you add an element to your fights out of the ordinary (Cool fight locations is another post) examples (more can be found in 4th ed DMGs<they really should have devoted more space to this>)
area is filled with lily pads or mushrooms that you can bounce on
individual squares on the map intensify and make easier crits or crit misses.
I had some very lazy and powerful spellweavers(an under love monster) have a bunch of very short range teleports and "moving sidewalks" on top of their ritual pyramid
patches of demonic spirits that hinder non-evil and aid evil
casting circles that aid arcane casting
unholy ground that hinders cleric(or the reverse)