Happy (and secret) co-owner of a NXT kit

As today, I'm the happy co-owner of a Lego Mindstorms NXT kit. The other shareholder is my son G., and he won't know about it until his 10th birthday next month. Will I be able to keep the thing in a box for so long?

I could use as an excuse that maybe he'll like better to get a working robot than to get a box full of parts... Or that it would be nice if a writing robot greeted him (at NXTLOG there are several samples that could get me started: Writer robot Final version, TurtleBot Drawing, Plotter-Bot 3000, plotbot).

Lots of learning ahead!!