Competition Prize?

I am launching a website to ask for a million positive thoughts. I would like to offer a special incentive to one or a small number of adults or children who submit their positive thought, and it needs to be something worthy of this - and not just money. I'm thinking of giving away a complete set of the Encyclopedia Britannica to one person whose thought is selected at random, and I'm concerned that this is a suitable gift - or would you be kind enough to put forward your suggestions?