Many students are active in high school - clubs, sports, etc. At university, you can still do stuff, but you have to be selective and probably dial it down. With the expectations of a university course load, it is best to start off carefully and then add activities as you figure it out! Extracurricular activities are great for a work balance, mental health, possibly physical health, meeting people, etc. We suggest that you start off with perhaps two activities - one that is a club or project type, and one that is intra-mural or physical.
We have seen too many students take on too much or assume that they can do what they did in high school. Remember that you will have heavy periods during the term - way heavier than high school!
Many programs, departments and faculties have student societies representing the students in the program.
There are intra-mural sporting activties within a faculty, student housing, campus-at-large.
There are clubs for a wide variety of activities and interests - some very casual like game playing, to debate clubs, etc.
There are program / inter-program / dept / faculty teams that compete locally or wider afield on various topics and they often need a team made of up folks with a variety of backgrounds and skills.
If you are interested in research and academic thinking, skills, you can approach instructors about helping them (they may or may not have funding, so assume it is volunteer and unpaid, and assume that it will be around 8-10 hours per week)
The campus student group/association usually has lists of campus wide activities and supported clubs.
At Waterloo, there is a site with club information.