I'm not just talking about being on the student council or joining the French club. I'm talking about going one, two, or three steps beyond the ordinary, to the realm that requires creativity and originality of thought to pursue your passions. For example, students who are interested in French might join the French club, but they might also look into spending a summer at the Canoe Island French Camp in Washington State (yes, they offer financial aid), spend a summer or even a year abroad as an exchange student, encourage their parents to host a French exchange student, or subscribe to audio magazines such as Champs-Elysées to improve their comprehension and fluency. All those activities will not only help the student be a better French student, but a French student with a broader worldview. And, as a secondary consideration, a stronger college application.
Let's try another, more general topic. Leadership is a key trait many colleges look for, and one that is specifically called out on the Common Application. When students list an extracurricular activity on that application, they have room to note whether they were the team captain, the vice-president of a club, etc. The implication is that they wouldn't get to be a leader without both a long-term commitment to the team, club, or organization, and the vote of their peers (although I am reminded of a tongue-in-cheek club at Stanford, whose description was simply, "We are a resume-building club; everyone is a Vice-President.") Students can find leadership roles in nearly any organization. To take it further, participation in activities such as the Rotary Youth Leadership Awards or the even longer-term ventures such as an Eagle Scout rank (for Boy Scouts) or Gold Award (for Girl Scouts) earns the student a double whammy of leadership and community service credit on their college apps (not to mention scholarship opportunities).
I don't mean to imply that these things should be done solely for the purpose of padding a college application. The primary goal is to help a student make discoveries about what their interests are and what opportunities are out there. The secondary consequence just happens to be a stronger college application, but that is because the individual will be stronger--more resilient, more confident, and more mature--for having these types of experiences.
My favorite source for finding amazing extracurricular and summer activities is EnrichmentAlley.com. You can search their free online database by topic, by region, by summer vs. school-year. Here is where students can learn about all sorts of opportunities to indulge very specific interests. Sports broadcasting camp, anyone? Pyrotechnics camp? Dinosaur Academy? Many programs offer financial aid, and for those that don't, creative students can sometimes apply to a local organizations for assistance in coming up with funding for special opportunities. But you'll never know until you go looking.