I think spending your time doing something that you care about, and that enables you to grow and develop your interest in whatever that is, is what you should do with your time. If you were a kid that is super interested in musical theatre, then than could be a job working at a musical theater summer camp, or maybe that would be volunteering for a local community theater. If you were super interested in a particular sport, then being a ref over the summer for that sport (volunteer or paid) would be great. If you have 12 hours/week to do . . . something . . over the summer, pick something that will continue to demonstrate your commitment to activities and interests you’ve already started to develop the past few years, whether it’s paid or unpaid. ND does like to see community service, but if it’s a choice between a “random” community service activity that doesn’t fit with any or your interests, or a paid position that does, I would argue that it would be better to do the paid position.
However, the main point of my post above is that ND is a reach school for every single applicant and it rejects 85% of its applicant pool. Many of the students it rejects are stellar applicants with top notch academic and extracurricular credentials. Don’t get hyperfocused just on this one school. Spend much more of your time finding other schools that are targets and safeties that you would love.