Waiting until the senior year to start making oneself attractive to colleges is one of the biggest mistakes a high school can make. Colleges like to see sustained achievement across all of the high school years, both academic and extracurricular. When an admissions officer looks at an application and only sees AP courses and outside activities in the junior and senior year, the admissions officer will likely think that the applicant was only doing those things because they look good on a college application.
While it's good for students to enjoy high school and not be thinking about college every single second of their lives from ages fourteen to eighteen, a truly college-bound student will not wait until his senior year to start thinking about his college application. Every year a student should be doing things to prepare for college.
Freshman year: it's unlikely that most students will be taking AP courses their freshman year and that's fine. Students should take high level (honors or their school's equivalent) courses whenever possible, but also should think critically about what activities to join. Students should feel free to experiment, but also should pursue activities they think they'll stick in for all four years, as admissions committees like to see sustained involvement.
Sophomore year: steady as she goes. Keep taking as high of level courses as possible, without overdoing it, focus on getting good grades (keep on memorizing those Hamlet quotes and that "y=mx+b"), and maintain involvement in activities, presuming they're still of interest and value. It's not too early to try to take on some leadership roles in those activities as well.
Junior year: here's when high students should really kick into high gear with those higher level courses. It's time to take AP Exams and run for treasurer of that club or captain of that team. In addition to sustained involvement, admissions committees like to see leadership roles and increased responsibilities in extracurricular activities as well. It's not too late to take on an additional extracurricular or two if the interest is there. No activity looks "bad" on an application, and students should always remember that high school is still their life, and not just a period of time to put on a form for complete strangers to judge and evaluate.
Senior year: the Big Kahuna. Students should maintain high level courses. By now, you should have already taken the major entrance exams like the ACT and SAT. It's important to take the tests as early as possible during the senior year (most students start at the end of junior year) so that the students have the option of retaking any test to achieve a higher score. Now's a great time to go after an internship or take a course at a local community college to display an interest in life after high school. Many students, even those focused on going to college, don't always see the bigger picture of their lives in high school, and getting involved in the community or taking courses outside of the safe bubble of campus is a good way to demonstrate maturity and ambition.