I have a good project in mind for ya! Put them in the mail, and see how much time it takes for them to arrive at my place! 
On a more serious note, I have no clue.. Depends on what you are into I guess. You could do home automation stuff with one xbee enabled arduino controlling stuff close to your computer, and another recieving commands from the first to control stuff elsewhere. Or do the XBee take up the serial pins of Arduino, so it can't communicate with the old 'puter?
If you had more Arduinos (and I knew more about XBee) I would suggest trying to design a master Arduino that sends out broadcast-packets to all XBees, with addresses corresponding with each Arduinos unique id so the Master could send commands and requests to the Slaves. Then, you could try some distrobuted processing doing hard maths like calculating prime numbers or some such. ^^