Unless you’re a sophomore, considering it takes about a year from contract to publication (in the best of cases) finishing à novel this summer won’t help with admissions. However it is a worthy achievement to finish it if you have a novel waiting to burst out.
As for international charity, frankly it’s more a negative. You’d be better off spending your summer helping a * local * charity and REALLY being helpful, full time, not just a couple weeks, but the whole summer, continuing for a few hours a week - although it’s a bit late if you’re thinking in terms of admissions and highly selective colleges won’t see it as as meaningful as volunteering you started before that summer. Generally speaking, things you so just fr college admissions aren’t useful since adcoms see through it. Do things because they matter to you.