I think different people enjoy different aspects at high school, so the answer to what is the best thing will vary from person to person.
Personally, I enjoyed being part of the school sports teams.
I played hockey in the winter and tennis in the summer, and as well as the physical act of running around and being outside, the social aspect was great. A lot of my best friends were on the teams with me, so it was always a great opportunity to have a good laugh.
On top of that, we got to travel around to different schools and different parts of the country, which was also great fun.
For other people it might be a different club, like theatre, or the debate team, or even the learning! At the end of high school you finally get to ditch some of the subjects you don't enjoy, and try things you are more interested in, like Media Studies, for example, which makes lessons a lot more bearable!