There are certain mainline quests that put you into a private instance on a small portion of the world map. For example, the Sylvari quest "Seeking the Zalisco." It's a low teens quest, but many will be similar. At the end of the portion where you meet with Speaker Mahuti, the active quest portion ends and you can leave. However, there's still a small army of green NPCs there and a whole bunch of neutral (yellow) fireflies and does.
First, equip a ranged weapon.
Then:
-Select a yellow mob.
-Attack with your autoattack 2 or 3 times. You'll know that the you'll get credit for the mob kill once it's name turns red.
-Move on to the next mob and repeat.
The green NPCs will swing by and kill the yellows for you, so you can basically run around this little area just tapping mobs and getting full xp and loot (since that's how guild wars 2 does things). The respawn is fairly fast as well.
For those of you who use Macrogoblin (bot already working), this is also a nice safe place to bot for a few levels since it's instanced and no one can see you.
This tip is less about the specific area, and more about giving you an idea of what to look out for for 'safe' botting.
Happy Hunting!
EDIT:
Oh... and for warriors, who have a buttload of weapon combinations, it's great for training those all up real fast. Since you get credit towards unlocking these abilities with the kill.