That's a great idea. My old guild used to do just for fun get togethers and it was a blast. We would just go on random fun quests to get rods of transformation or bootleg flasks, or other fun and useless items. We also rampaged around the world as a band of killer ninjas, fire breathing bears, and other crazy stuff that was just a lot of fun. We had barbecues and fireworks shows and just random hanging out parties. It not only makes the guild a lot more fun and close knit, but it also attracts new members when they see how much fun the guild is.
About the location, one nice thing about our guild is we never seem to be in short supply on locks. So as long as 3 of us can reach an area, the locks can summon the rest. So it's even possible to get together on one of the floating islands in Nagrand, or atop a giant mushroom in Zangarmarsh, or just someplace beautiful like the Astranaar bed 'n breakfast...
Actually I had a bad experience there. I was just going up to my room after a long day of questing and BAM, lv 60 horde rogue jumps out of the bed and backstabs me to death. Hopefully they have fixed the horde infestation by now though.