Start with a deep understanding of your users. What might interest
them? Where in the interface are they likely to take time to explore
something further, and where do they just need to get something done?
Create "doors" into the supplemental content that would appeal to users.
These doors might be underlined links (even in desktop applications),
headlines, buttons, menu items, icons, or clickable image regions
-- it's up to you to figure out how to label them in such a way
as to inspire curiosity. There's an art to it. When in doubt,
usability-test it with a representative sample of your user base.
With particularly obscure affordances, like icons or images, you
might want to add tooltips or some other kind of short description
to inform the user where they might be going when they click on it.
(With an Easter egg, though, its very non-obviousness is part of
Also, provide an obvious way for the user to get back to their
original workflow. The idea is to persuade users to read the branch
content, and then go back to what they were originally doing; don't
get them stranded in a backwater! Pop-up windows should provide
"Close" buttons, and new pages in a browser-like UI should provide
"Back" links or buttons.