Look on pintrest.com!! They have great ones! :)
I seen someone on here before say they bought a bib that said grandpa or grandma, or whoever they were telling and thought that was the cutest idea
My friend just joked about them being grandparents while dinner was cooking then at dinner she pulled out a pic from the first ultrasound and said congrats to the soon to be grandparents
Good luck! Tell us what you do!
Tell them you were trying to remember something from when you were little (a funny story or something) and then smile and say "good, I wanted to get my story straight for when I'm telling our little one stories about me!" I did something similar with a baby blanket to tell my grandma. :)