A banjo would be very classy compared to some of the stuff people leave there (mostly beloved old action figures and plushie Cthulhus, I admit). I left a pebble there last year. It's a beautiful cemetery, incidentally.

Unfun fact: every few years, some new band of jerks decides it would be a wonderful prank to dig up HPL. Hitherto, they've all been foiled by the cemetery guards. (Stones are all changed now in nine grounds out of ten--negothick has speculated that they may have moved his coffin away from his headstone to prevent anybody finding it in a future graverobbery.