Thanks for being such a great part of the forums here—that means a lot.
This is my opinion about premium releases: Spend most of your effort and energy on the slip. Then spend most of the remaining time on the booklet—essays (hire a proofreader!—I'm a freelance editor/proofreader.) and behind-the-scenes details are much more valuable than just stills from the film. Then a little time on the other extras—cards, etc. (though I personally don't care very much about cards, and wouldn't miss them if they weren't there). And if any time remains, go ahead with the extra gifts. Probably no one is making a purchase decision based on an extra gift, and as you say, it's hard to find something you can include that's cool, relevant, and not very expensive. So don't worry too much about it. The heart of any premium WWA release is the slip. In addition, anytime it's appropriate to use concept art or something just a little out of the mainstream official poster art, I think that can make a great release. (Check out some of the stuff that's included in the World Cinema Library's upcoming 2001 release, for example.)
Just my opinion—hope that helps, and thanks for asking!
To be able to match Filmarena at this early stage in your production is fantastic—congratulations!