King's Quest III: To Heir is Human
Look behind the tapestry on the wall next to Gwydion's room to find a reference to King's Quest IV.
A glitch rather than an Easter egg: while falling in the pirate ship, type "jump ladder" and you'll be able to walk on thin air. SAVE first!
Drink from the water of the waterfall in the mountains to get the following message.
When talking to Rosella, she'll ask to see Alexander's birthmark on his butt, which he embarrassingly does.