from what i remember this is a basic list of bonuses:
old world = 0 xp mod.
old world indoor zones (dungeons like guk and befallen) = 15% xp mod
all expansion zones = 15% xp mod
all expansion indoor zones (dungeons and what not) IIRC 12% on top of the standard 15% from being expansion zone.
some zones are higher and lower, but those are the rough mods.
so for easy xp hit old world zones as the mobs hit for less, have less xp, have less resists, and die faster, but you get no xp mod unless indoors.
this is why PC has such a mega xp bonus. it gets full expansion and full indoor bonus, plus zone bonus. some zones have bonuses above and beyound the standard mods.
low 40s try hitting CC. you will get a great xp bonus there as its expansion and indoor. mid 20s try NL (neth lair, that tunnle place just off nexus) for more xp bonus, but i found LDoN best for 25-35xp.
im sure someone here will have much greater detail on the zones then i provided, but that is just a basic outline.