Refactor a ledger printer.
The ledger exercise is a refactoring exercise. There is code that prints a nicely formatted ledger, given a locale (American or Dutch) and a currency (US dollar or euro). The code however is rather badly written, though (somewhat surprisingly) it consistently passes the test suite.
Rewrite this code. Remember that in refactoring the trick is to make small steps that keep the tests passing. That way you can always quickly go back to a working version. Version control tools like git can help here as well.
Please keep a log of what changes you've made and make a comment on the exercise containing that log, this will help reviewers.