Well, I suggest using something affordable, like domain names are always useful, gift certificates is a good idea, advertising, something like that.
You can go for CDs, DVDs, books and other entertainment merchandice.
I would also recommend that if you have further promotion questions regarding non-hosting related sites, publish them at www.PromotionTalk.com
forum. Since it would be getting more attention and attracting more promotion and advertising experts, you can get a complete coverage, tips and useful suggestions on how to get where you want your site to be.