I am confused by "later". Are you writing about the past or the future? Past - one event: Many years later, I drove my own children to Granny's house and the first thing I did was get/to get us all a cookie. Past - habit: Many years later, I would drive my own children to Granny's house and the first thing I would do is get/to get us all a cookie. Future: In many years time, I'll drive my own children to Granny's house and the first thing I will do is get/to get us all a cookie. The meaning of "the first thing to do" = The first thing [subject] has / will have to do. In your example, it is better to write "I will do".

Hey there! "to get" because an infinite form is needed because you have "do" as an infinite form. I also agree with Michael. "In many years time..." is better.