I am not sure what we thought about-maybe what we would do as a grownup, whether we even wanted to grow up (I think we did, as the grass appeared greener on that side then). Maybe about doing wonderful things like flying (my first flight was at age 23), travelling to a different country or countries (that happened on my Ph.D. cum sightseeing trip at age 26).
Most likely, thoughts also included finding the love of your life (maybe not, maybe you thought you had already found her/him with every new crush), owning your own car and going on an adventure (it IS an adventure, come to think of it, on our city roads), owning a Badminton court so you could play without interruption ( a friend from college in Hyderabad had this ambition of having a Billiards Table in his basement and playing with Me!).
The world of thoughts is not constrained by reality, and is positive at all times. To this world!