I like the idea of visualizing the parse tree, and another fun place to go would be to do a real IR, bring things into SSA form, show phi function placement, and then do register allocation and optimization.
I have in the past used xdot files to good effect for visualizing phi function placement and interference graphs. What would be great is a way to make the result interactive (e.g. drill down from basic blocks to instructions, selectively get more or less detail on voluminous data like live ranges). Graphviz unfortunately isn't suitable for that.
