I like to change the guild announcement to new and interesting things. We now have a "quote of the day" so surfing the net for strange quotes (heh, or from screenshots/memory) is fun ... sometimes we play "guild hide and seek" and go stand somewhere in a town or outpost (always dist 1 to make it fair) and make the rest of the guild look for you. Scavenger hunts, etc. I am fortunate to have a very fun-loving guild
Strangely enough, I like to visit the more spammed areas (Ascalon, LA, CA, Droks, etc) and call out scammers, or just banter with people in local chat. There are some very interesting people i've met this way, as well as some real jerks (see the "Biggest Jerk you ever met in guild wars" thread for that stuff). For example, take those people that like to just walk up to you and offer a trade, then you decline, and they do it again 10 times. When you eventually accept, and they offer you some horrendous item; just find the worst item in your inventory and offer it back, they decline, then chase them around the district hitting the trade button (like they just did to you) until they get frustrated and leave.
Sometimes I make really absurd or annoying builds for play in CA (touch ranger, anyone?), with odd names (nothing that'd get me in trouble, of course) and just fool about for an hour or two.
And of course the usual stuff mentioned earlier in the thread: help out new folks in ascalon city, missions, etc.
last week I helped folks do the great northern wall wearing nothing but a santa hat and my bow
Streakin the beacons is fun sometimes too, although there are people out there now that stole that idea from me >_<