PhD programs in mathematics, statistics, philosophy, and theoretical computer science tend to give you a great deal of free time and flexibility, provided you can pass the various qualifying exams without too much studying.

Economics also has good opinion among the EA/rationality crowd: - http://noahpinionblog.blogspot.co.uk/2013/05/if-you-get-phd-get-economics-phd.html - https://80000hours.org/career-guide/top-careers/profiles/economics-phd/ - and posts by Bryan Caplan (links under the first link)