This is a great article posted on another site (who reposted it from another site, who took it from another), but I thought many would enjoy it here. It is The Lazy Man’s Guide for Constructing a Call of the Cthulhu Adventure, written by Sandy Petersen, original author of the Call of Cthulhu.
The lengthy process to do something necessary or a magical scroll needed is inscribed on the other side of a great tapestry and can't be removed. For more fun it could be in the kings throne room. Try to hide in shadows with a 20*5 (yards/meters) monstrosity!