Read the small print (i.e. the what to take section of the dossier, including the optional items). On other trips I've done this has been a 'nice to have' list, but for this trip when it says essential it means essential.
Temperatures varied from sub-zero at night to mid 30s in the daytime sunshine. Water purification tablets and toilet paper are essential, and a big pack of wet wipes proved a welcome addition to my pack on numerous occasions. When it rains, its a tropical downpour - decent waterproofs are a godsend if the heavens open.
Remember a $20 bill to pay for your visa on arrival. The embassy website said bring 2 passport photos, but we didn't need these as they took digital pictures as part of the process.
Practice your dancing and singing for the last night social with the trek team. There's got to be something better than the hokey cokey......