Interest in various extracurricular activities will vary from one college/university to another, so it's virtually impossible to say, in general, what the most popular extracurriculars are. On the webpage of each institution or in other impartial evaluations, however, there will often be an indication of what activities are particularly popular at a given school. Although this would surely not be the most important factor, this information could be part of the puzzle In making your college decision.
Activities at most schools will run the gamut of just about anything that a person could be interested in doing - sports, art, music, dance, theater, community service, student government . . . etc., etc., etc., the list goes on. Quite often, if an activity doesn't exist on your campus and you'd like to start an organization focusing on that activity, you can do it and get some financial support for the project. Someone wanting to start a club or activity usually just has to present the signatures of a certain number of people who would like to see the activity offered, and the number of signatures required may be quite small. This process is often handled through the student union offices.