Military Rank (http://st-www.cs.uiuc.edu/users/johnson/cs497/military.html) A simple lesson in writing a concrete subclass of an abstract class. The abstract class in this case is Magnitude.
Money (http://st-www.cs.uiuc.edu/users/johnson/cs497/Money.html)
Gives practice for double dispatching (well, it does in VisualWorks, but not in Squeak) and seeing how Numbers are implemented.
PeanoNumbers to explore the power of polymorphism.
The Collection Homework (http://st-www.cs.uiuc.edu/users/johnson/cs497/collection.html) Gives practice with collections, which is important for any Smalltalker.
FunWithStreams streams and component programming. And shows that some components can be used in non-obvious ways.
More to come.