An active essay is an essay on a computer that contains not only text, but some form of computerized text. Unlike a printed essay, an active essay can have animations, buttons, calculators, and even full-blown simulations built in to them.
A major goal of Squeak is to allow the creation of active essays.
In the sense of DTSTTCPW a workspace with formatted text (different colors, different sizes for headings) and code actually serves the purpose of an active essay.
Or a book in the help browser with code snippets.
Or Squeak wiki pages accessed through the HelpBrowser.
Why not write an active easy about something you find interesting, and post it on SqueakMap and/or the Swiki? The people on the Squeak Mailing Lists and in IRC channel will be extremely helpful with any difficulties you run into.