For ONEPIECE fans, I found this the other day. If you ever plan to stay in Tokyo during that period, you may want to check it out!
There’s
an official website for ONEPIECE ten (ten means exhibition in Japanese). However, it doesn’t seem that they have an English page, unfortunately.

The basic information is like this:
WHEN?:
From Mar. 20, 2012 to Jun. 17, 2012 open everyday from 10am to 10pm. (But you can only enter the time you chose at the purchase of your ticket.)
WHERE?:
Mori Arts Center Gallery, Roppongi Hills, Tokyo (52nd floor)
TICKET:
To be purchased at 7-eleven in advance. (6 entries per day, so choose the entry time when purchase a ticket. See here. No refund, and no changes of the entry time accepted.) This system is similar to Ghibli Museum ticket although it was from other convenience store.
HOW MUCH FOR THE TICKET?:
1800yen for adults and college/university students
1300yen for high school and middle school students
500yen for age 4+ and elementary school kids.
(You can bring your baby but no pets are allowed.)
WHEN TO BUY THE TICKET?:
Entries from Mar. 20 to Apr. 5…already purchasable.
Entries from Apr. 4 to Apr. 30…already purchasable.
Entries from May 1 to Jun. 17…starts selling from 10am Mar. 31

WHAT CAN YOU SEE THERE?:
Original drawings from best scenes, special movies, interactive arts and etc. You can also buy some special goods there.
REMARKS:
-It’s prohibited to take photos and record videos at the exhibition.
-You may be asked to wait outside, depending on the situation. (You’d better bring your umbrella with you, just in case of rain.)
-Don’t eat or drink. No smoking.
-There is still a chance to buy a ticket at a slightly more expensive price than the advanced ticket at the exhibition site on the day, if only you’re very lucky. But that’s limited to the case when there are some unsold tickets left.
I’m not a fan of ONEPIECE but I recognize it’s quite popular so I thought it might be nicer to announce such event for those who plan to stay in Tokyo during that time. :p