What are the most valuable skills/certifications an 8th and 10th grader can start learning over this summer break?

I am writing to seek some advice for my nephews who live in the USA. One is in 10th grade and the other is in 8th grade. They are about to start their summer break, which lasts for almost two months.Since we don’t have many connections there and aren't fully aware of the best opportunities available, we are looking for suggestions on how they can spend this time productively. We want them to learn a new skill or at least start something that will benefit them in the future. Could you please suggest some productive activities or skills they could look into during these holidays?