One huge advantage to using Greenfoot is the visualization and interactive nature of the application. Using and demonstrating interfaces will take some work though.
Anything that gets added to the world will be displayed and interacted with as it's actual type via the context menu. This can be confusing as students will be able to execute (via the context menu) all methods that are in its hierarchy: Object, Actor, ASubClass, etc. Even if the declared type is an interface.
On the other hand, when working programmatically the coder will be restricted to the methods defined by the declared type (the interface). This is very useful and worth demonstrating via code completion (ctrl+.) and compiler errors.
Using interfaces add clarity to the design and responsibility of classes in an application. This is a great lesson for students.
In a Greenfoot application most visible objects are Actors. In many applications programmers are making use of the World/Actor mechanism (by subclassing), yet these classes often have their own unique behavior and semantics that are separate and distinct from the Actor class and are only Actors to leverage the Greenfoot World/Actor architecture.
You will see these issues clearly if you create any scaffolded exercises or simulations of your own.
I use Conway's Game Of Life as a project for a few labs. The logic behind the simulation is very easy to understand and implement, and this allows the student to focus on design issues that interfaces help with. A cell can really only live or die; is represented by an Actor subclass; but should never be able to behave as one, ie move or rotate. See Demo.
Karel J Robot is another good simulator that can used for labs (also available to educators via GreenRoom), as can any tile-based game or application. The next demo is the introduction to interfaces lesson from the text book A Gentle Introduction to Object Oriented Programming with Karel J Robot where each contractor implements a Worker interface to build a part of the house. See Demo