Here's a great tip from Darthkurai on LJ: doing the Midsummer Fire quests as a lowbie will net you a ton of nice XP. He grabbed almost two levels' worth of XP at level 40, just for running around to all the different bonfires throughout Azeroth. Bewarned that some of the quests have various level requirements on them (Undercity and all of the capital flames, as he finds out, are level 50 and above only, and of course Lord Ahune is only for level 70s), but doing all of the quests at low levels will net you a good 6-7k XP, which around level 40 will give you at least a ding or two.
Pretty good for just running around the world once or twice. Everyone's excited about Ahune and what you can get from him, but don't forget your old alts, either -- logging them on during the festival and showing them the world (while killing on the way with those buffs) can grab you a nice chunk of free XP for them as well.