..... I'd discover the answer to each and every question/mystery/truth of the world in general
I would take very very detailed notes about history before getting back on the Tardis.
I'd hang out with my 18yr old mom and see my mom when she was a kid. With my 18yr old mom we'll go to the club and dance to 70's & 80's music(she loved to dance, she said she always went dancing during weekwnds, it sounds fun), and then she'd introduce me to her friends when they were young and we'd all go out and eat somewhere and have a nice conversation. With the yonger version of my mom, I'd see what she usually did with friends when she went outside and meet the rest of her family(a family of eight!) and learn what they were like(some of them already passed away and some live far away so I've never met them). I'd especialy meet her oldest sister. She passed away when she was young and my mom always talks about her when she recalls her childhood). I'd have a splendid time!