Although Bill Watterson famously was opposed to merchandizing of Calvin and Hobbes, I just learned from Wikipedia that there was a USPS postage stamp featuring C&H, honoring it among some other noteworthy newspaper comics.
So, if you're ever looking for a nice little gift for a C&H fan who has all the books, and who isn't too opposed to moichandizing... there's a stamp-collecting thing called a "first day cover", in which a stamp had been put on a postcard, and canceled with the date that the stamp was issued, specifically for collectors. (Sometimes the cards have additional art, and sometimes the cancellation mark does.) These first day covers are affordable enough that you can just put one on the fan's fridge, for them to find.