Well, that depends on what exactly interests you. On the whole, for a modern English speaker, there is a huge wealth of 18th and 19th century literature available for free online, which is probably optimal because it's distant enough to offer interesting perspective, but at the same time highly readable and not so alien as to be incomprehensible without specialized study. On almost any subject that is a matter of ideological controversy today (or that was a matter of controversy in past ages), you can have a lot of fun by reading through random titles from past centuries that come out of Google Books when you search for the relevant terms.
A Wall Street Journal article by Harvard professor of government Harvey Mansfield claims that the social sciences and humanities are inferior to the sciences. The article implicitly urges undergraduates to major in science. From the article:
Do you agree with this? As a game theorist I probably have a rather biased view of the situation. It's certainly true that the ideal of the scientific method is vastly better than the practice of economists, but I think that majoring in economics provides better training for a rationalist than majoring in any of the sciences does.
Economics explicitly considers what it means to be rational. Although it infrequently considers ways in which humans are irrational, I'm under the impression that the hard sciences never do this. Furthermore, because economists can almost never perform replicable experiments we have to rely on what everyone in the profession recognizes as messy data; therefore we’re far more equipped than hard scientists to understand the limits of using statistical inference to draw conclusions from real world situations. Although I have seen no data on this, I bet that a claim by nutritionists that they have found a strong causal link between some X and heart disease would be treated with far more skepticism by the average economist than the average hard scientist.