Not sure I fully understand what you mean by //They're too old.// Too old for what? Perhaps take them out for a special meal? Nice to be treated and waited on now and again.
Where are you? Here I would take them out for afternoon tea at a hotel that offers that ... or provide an afternoon tea for them your self...even more special if someone you love does it for you...google 'afternoon tea' in London and see what is offered...generally cakes, sandwiches, a small dessert.....aglass of fizz maybe.
Take them out. Spare a few hours of your life to celebrate the long marriage and lives of your grandparents - it’s a small thing, but will make them happy. Emotions are extremely important for elders, don't forget about that.
And if possible, you can make some photos with them so you could print them later. It would be a nice family evening! Good luck and congrats them on their beautiful marriage.