Where did you meet..was it quirky? You could talk about still meeting them there in 5o years time.
What is their biggest achievement? You could talk about how much you love that about them.
What is their love language? Talk about this during your vows. If it is touch say that you hope you are holding their hand in 50 years time. If it is quality time, you could talk about still spending loads of time with them in 50 years time. (If you don't know your partner's love language, you can find out here).