Find a small static dataset that you can put in JSON, CSV, TSV, etc...
Use D3 to visualize this in an awesome way. Be creative, explore and don't cheat yourself of a good chance to really dig into D3 tonight.
Additionally: Comment your code to show that you really understand what's happening. Write an explaination.md file that explains your overall logic.
Don't use Rails. Don't stress out over animation (as we'll cover that tomorrow) unless you really want to.