my grandparent's 50th anniversary is next week...?

yes. take a nice piece of paper and write, dear granpa and grandma,

and then tell them a story that starts " when i was ... i remember how special it was to come to your house and.....

and tell them a story of how special they are to you. what you recall about holidays, about things they have done for you or for your family, how they are loved by everyone in the family and how happy you are that you get to share their 50th anniv with them. and that you hope they have many many more.

this will probably be a whole page. after you finish, fold it up and put it in the card. sign the card.

a 50th deserves a letter in a card.

You have been given some wonderful suggestions here. I agree that something from the heart is a great gift. You might want to write them a little poem or song. I've written poems for a few of my nieces and nephews when they graduated and they said they really liked that. Happy 60th to them!