The first thing I'd like to say, Don't Sit Idle!

Thats a mistake i made during the summer vacation after my own 10th std. It's amazing to see you want to do something in that time. And though i appreciate this, i'd like you to ask, Are there any hobbies or passions that you have you'd like to pursue? IF so, you don't need any course, you should just go and do your own thing. Because contrary to what you may be thinking now, you can't learn the guitar, or learn to animate videos while preparing for competetive exams during high school, or even later in graduation. So do it now!

But, if you're sure you want to be productive then you have the following options :-

1. Study. Yes, you can skim the books of 11th std and get an idea of what's ahead. It'll also help you further decide what  stream you want in high school.

2. Learn to code. This will be amazing if youre  even remotely interested in coding.

3. Study Maths, if that's where youre headed. Trust me, maths is everywhere, and it won't hurt to be master at it.

4. Go to sites like Udemy, Edex, etc, and skim through the courses offered, enroll into a course if it interests you.

You cn take up basic technical courses like MsExcel, PowerPoint, SAP Modukes, Communication skills which are highly  important in todays competition.

You can check onlune course providers ljke Udemy, Udacity, Simplilearn which have abundant courses for beginners and provide certification with tremendous jib opportunities.
All d best.