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".