By playing a little "True-Or-False" quiz on the "Tokyo Metro 10th Anniversary Special Site", you can unlock a neat papercraft Tokyo Metro 16000 series subway train to download! ;o)
The quiz is in Japanese, but since it's a true-or-false quiz, you could guess your way through it if you don't understand Japanese (a red "正解" after choosing your answer means the answer was correct, a blue "不正解" means it was wrong - or you can look at the answers at the bottom of this post);
If you got all 6 answers right, at the end you will see a blue button near the bottom saying "プレゼントをダウンロード" that will let you download the PDF parts file (otherwise it will say "もう一度チャレンジ！" and take you back to the beginning so you can try again).
(note: because it says it's specially for April, I don't know if it will change every month maybe...?)
Good luck and have fun!
(Tokyo Metro April 2014 Test answers: Q1x, Q2o, Q3x, Q4x, Q5o and Q6x)