They're taking up the largest section of the International Space Station yet to be delivered.  The "Kibo" pressurized section, which is as large as a "tourist bus" - the Japanese portion of the ISS is quite amazingly large and complex.  This section is so large that the extra boom used to check for damage to the shuttle had to be left on the ISS LAST trip!  The Japanese contribution will be taken up during THREE trips of the shuttle (one part is alredy there!!  The Japanese portion has its own robotic arm, its own external landing to perform experiments - and the big portion, shown here being packed up (the below picture and much more can be found here at NASA):

  

Not that this trip is over, but the next Space Shuttle trip is the Hubble "fix-it" - once deleted but back by popular demand - thank you all!!  Left out because there is no way for the Shuttle to get help once on the way to Hubble!  However, to just leave such a wonderful piece of hardware (the telescopes) that has done so much and is still producing so well, would be almost criminal!  Looking forward to that September trip too - Hubble ESA site is here!