Well... there is the stock market, but that's generally too much of a challenge; any edge you get disappears very quickly, so the best thing to do is "free ride" off of other people's attempts to value stocks and just buy index funds (or the equivalent).

Other domains in which rationality can be tested are "intellectual sports" such as poker, chess, or Magic: The Gathering... it's hard to test "rationality" in a way that doesn't simply test intelligence or learned skills, though.