Offline wallet is probably best option. You may hide it in some encrypted place or your granddad's basement. yeah. noone would care to look for that.
Just leave some kind of clue to your relatives or the beneficiaries of you Will, for them to find it... I wonder how many Bitcoin has been lost, because people left a piece of paper
somewhere and their relatives did not know that it was loaded with money. That piece of paper should have a note attached, saying "To whomever are reading this... This piece of
paper are worth a lot of money, go to www.*********.com and find out how you can claim this." Point them to some sort of resource, where they will be able to find out more
about Bitcoin... And remember to tell them to keep the Private key safe.