I believe pbk (at my school, at least) also takes into consideration the type of courses you've taken while attaining your high GPA. They value breadth of coursework. Usually, at my school, if you majored in the humanities or liberal arts, this isn't an issue. But if you did very science-heavy stuff and didn't go beyond the humanities requirements, then you may not be considered for membership.
In the end, if you don't have anything more meaningful to put in your activities list, I would say put the pbk invitation there. It's an honor. Yes, adcoms will be able to see whether you took a diversity of courses regardless of PBK membership, but having that pbk distinction shows that you've been vetted by your institution and qualified for an honor that isn't dished out to every student who makes a GPA cutoff.