Everybody has that night where people do not show up and you have to cancel a raid before it really gets started. There are several activities to do on those nights where the raid is canceled. It also depends on what raid is canceled.
Last night, our 10 man raid was canceled due to us missing a tank. So what exactly did I do. Nothing really. I watched the new episode of South Park and Ugly Americans and went to bed. But that is just what I would normally do on Wednesday anyway. However, if I was going to list some activities to do, this would be a list of what I would do in WoW.
1) Run a random 5 man - I mostly avoid these now because I hate dealing with PUGs. However, I do have to remember some of my tanking skills so I throw on my bear gear and tank a 5 man. Just one though unless I need emblems for gems, etc.
2) Run a random BG - The rewards are pretty good for running the random BG now so I would do that and get some honor for PvP gear or gems.
3) Run a QQ Premade - I mentioned them in another post, but this a group that runs alliance 40 man BG to get maximum honor. You do not even have to actually join a group. You can join up individually from anywhere. Just jump in their vent and good to go.
4) Level fishing - I'm 41 points from max fishing so I could finish that before Cataclysm.
5) Druid Sky Diving - A little sport I invented in Burning Crusade. This is a lot of fun to do if you just want to mess around. Jump off of Dalaran yelling "GOODBYE CRUEL WORLD" only to be safe and flying off near the ground.
6) Run a 10 man - If the 25 man raid is canceled, we can usually get enough together to do a 10 man. From there, the possibilities are endless. Heroic ICC 10? Sure. TotGC? Awesome. Ulduar HMs? Great.
7) Alts - I have a mage who is level 73 and is so close to being at max level. I would really like to get my mage to 80 before then. Then, I would like to get my level 61 deathknight up to level 80 by tanking heroics. I'm even thinking of doing the Recruit a Friend again to get the new two person rocket. Then I can go around to people in the guild and tell them to jump on my rocket. :)
8) Crusader - I'm both surprised and not surprised at how many people don't have the Crusader title from the Argent Tourney. On one hand, it is almost 30 days of grinding to get it plus you may have to go back and do starter area quest to raise rep with some factions. However, it is easy to do the quest and you get massive amounts of gold and other goodies. Plus, the pet you get for getting crusader that can act as a bank, a traders, or a mailbox is awesome. I already have this but it is awesome to have.
9) Mess with people in trade chat - Warning: Do not do this unless you are absolutely bored. Trade chat has been known to cause loss of brain cells when exposed for more than 15 minutes.
10) Play other games - I'm only 10 hours into FF XIII and have not touched God of War III. I also am in the Starcraft II beta and have played some, but definitely need to play more.
I'm sure I can think of a few other things, but would love to hear some suggestions on what you like to do in WoW when your raid is canceled.