if youre looking for a break from FFXI and not a whole new MMO... Do not go for wow unless you grab a free server with a somewhat enhanced rate. It does no good to break from a grinding MMO to get into another grind. If its casual play you want, check out free servers for any game you have interest in. A simple google search will bring them up for you.
There are a ton of MMO's out there, you just need to find one that can hold you for a few weeks.