I can imagine a site-blocking tool where you could select a browsing "mode". Each mode would block different websites. When you open an unknown website, it would ask you to classify it.

Typical modes are "work" (you block everything not work-related) and "free time" (you may still want to block the largest time sinks), but maybe there could be something like "a break from the work" that would allow some fun but keep within some limits, for example only allow programming-related blogs and debates.