I agree with that lovely idea of a pen too. Maybe gift it in a box - could be shoebox or wooden memory box (depending how much you want to spend with small keepsakes  such as letter(s) for each grandchild in separate boxes, CDs or other recordings of your voice, etc). When I lost my grandparents I had only a single photo for two of them and one letter from another as I lost them so young. My photo isn't even an original but a scanned copy with fading ink. Even their own framed photos (wrapped in tissue to avoid spoiling) in frames of your choosing, something that is theirs and does not rely on someone else deciding to give/or not. Whatever you are up to arranging. Sending good wishes to you and your family. x