What's your favorite interface in adventure games? Doesn't necessarily have to be the most perfect or easy to use; just the one that you're the most partial to.
Off the top of my head, a couple of low-hanging fruit suggestions (feel free to add more):
LucasArts "9 verb" interface (Monkey Island 2, Day of the Tentacle, etc.)
LucasArts "verb coin" (Monkey Island 3, Full Throttle)
Sierra "icon bar" (King's Quest V, Space Quest IV, etc.)
Revolution "Left does/right looks" mouse buttons (Beneath a Steel Sky, Broken Sword, as well most Wadjet Eye titles)
Mine is actually the one in Leisure Suit Larry 7. You click on something and up comes a contextual menu of appropriate verbs. If it's a door, you can "open" it; if it's a button, you can "press" it; etc. — and it also has an optional text parser for inputting your own verb.