With hallow's end in full swing I had time to take a look at the various things you can do during the festival and in an effort to maximize my xp gain for my lower level characters I parked everyone in Brill (Tirisfall glades).
Brill? That's right... Brill. The primary reason for parking all my characters in Brill is that the headless horseman regularly swings by and sets the place on fire. The reason I picked Brill over Razor Hill (where the horseman also roams) is that Brill is generally a bit more populated (on my server) and the fires that the headless horseman throws around can quickly be limited to just one building (meaning everyone piles on the same target).
But I am getting a little ahead of myself here... the point of participating in the hallow's end festivities with all my characters are based around the easy XP of 2 daily quests that both take place in Brill (or Razor Hill if you are so inclined).
The two dailies are as follows: summon the headless horseman and help put out the fires by tossing water buckets at them. If you put out all the fires the quest is completed and the headless horseman will spawn. If you then kill the headless horseman (apparantly by caughing in his general direction) he'll leave his head behind which is a simple clickable object that gives you another quest that you can turn in right there.
That's basically 2 quests for the price of one each netting about 0.75 of a bubble in terms of XP (scaling based on your level) and a nice set of hallow's end candy, a broom, maybe some toothpicks, a mask or whatever other stuff you can possibly imagine dropping for hallow's end.
Notable here is that the horseman's head will stay behind for a while (seems to be about 5-10 minutes) after which the horseman can be called again.
This means you'll actually have to stand around and wait for the pumpkin head to despawn so you can complete both quests on each character. If you're greedy you can also skip the putting out the fires part and simply get the second one by clicking on the left behind head (that's where the gifts are).
Fighting the fires isn't difficult but it has to be said that it's a group effort. You'll need at least 3 people to get the fires under control and beat the hidden event timer but more people definitely make it easier.
That said you can simply click on the water barrel in the middle of town which gives you a bucket of water. Clicking on the bucket of water gives you the typical AoE targeting indicator and you should then toss your bucket at the fire. The bigger fires spread fire further around so it's definitely worth it to aim your buckets at the biggest chunk of fire you can find.
About 1 in 20 people have figured out that you can toss the bucket to someone who doesn't have one meaning you can create chains of tossing buckets from person to person and get the fire under control a lot faster that way. People however don't read tooltips and only posses a modicum of cooperative spirit so don't expect this to happen much.
If you have travel form or a fast broom or something that'll make you run faster, use it since it will greatly reduce the time needed to get those darned fires under control.
At 1.5 bubbles for the 2 quests and hallow's end going on till the 2nd you can hope for about a level's worth of XP for the entire holiday for each character.
Combine this with the other quests (like raiding the candy bucket at the inn in each town) and you're looking at an easy 2 levels worth of XP at pretty much any character level since the XP rewards for the quests scale depending on your current level.
As a side note: the achievements can be tracked via the achievement window and can be very helpful in determining what candy buckets you raided already.