Its pretty rare you get someone that good. The basic renewal is a challenge for most of them and the PFL always turns into a lesson in itself. You could just sit back and enjoy a pleasant ride with a competent pilot. However, no-one knows everything so to spice it up ask those really hard questions like
1) will an aircraft roll faster at altitude or sea level
2) describe the five instrument errors (or six or seven if you feel mean)
3) why is orange jam called marmalade
4) explain ATSOCAS
There's always something more to learn. Your job is to facilitate that learning.