Original post by lordmasterluYour current extracurriculars sound great! There is no "checklist" because it's not necessarily about what/how many extracurriculars you do, but more about how you engage with them and what you gain from them. So the schools will want to see you demonstrate qualities like leadership, teamwork, perseverance, time management, etc. They should basically just be activities that you genuinely enjoy participating in outside of the classroom - don't go joining 10 different clubs you don't really care about this year, an admissions officer will see straight through that. I think it's great that you want to start a psychology research blog - it shows passion, initiative, commitment, motivation, so many things!
As for SATs, it's scored out of 1600 and half of that comes from the math sections (the other half is english reading & language). I would aim for a total score of 1500 at least, so sharpening your math skills is important. It's really be up to you how you prepare for it/how much time you spend preparing, because you know your skills better than I do. You can buy a textbook that usually has tips and tons of practice questions, as well as full practice exams (doing practice exams is a great way to prepare!) If you're confident that your grades will not drop, then by all means, concentrate on preparing for SATs. However, standardized testing is only one part of your application - it is definitely not worth sacrificing your academic grades to prepare. Also, keep in mind that you can take SATs as many times that you like, so you do have a chance for a do-over if you're not satisfied with your score the first time around. I don't think there's an actual limit, but you will probably not want to take it more than three times... it's a long and brutal test... You can also look into taking the ACT, which has four sections (language, math, reading, and "science" which is really just data analysis)
You can definitely take French as a subject test! A subject test is just meant to demonstrate your proficiency in a specific field, and it's completely up to your choice as to which ones you will take (you'll probably need at least two tests though, and some schools have specific requirements for subjects depending on the course you're applying to, so look into that)
One more thing, applying to Ivy leagues is a long and costly process, between standardized tests, and all the essays. It takes more effort than UK schools. I'm guessing that you'll be applying for 2019 fall entry, and it seems far away, but you don't have that much time. The application deadline will be beginning November 2018 (for Early decision) and beginning January 2019 (for Regular decision)