system
A chat on 4chan
human
I've about this question at least once a week for years. So long in fact that some of the times when I was asking myself this question would probably be times I'd want to go back to now.

I think it'd be the most fun to live the rest of this current life, and then go right back to the start of it with all my memories in tact. Well, maybe skip a few days in so I can skip the (probably traumatic) experience of being born. I like to imagine that I'd still have that insane early childhood capacity for learning, but with an adult's sense of determination. I'd learn several languages & musical instruments before I was 10 years old. I don't know how far I'd push it - being considered a child prodigy could be fun, but it might also draw unwanted attention. There would be a huge temptation to reveal my true state to someone by 'predicting' a major event like 9/11. No doubt I'd want to memorize all the most successful stocks of the last few decades before I went back.

Sometimes I think that this is actually what's happening - each time you die you go back & relive your life again, but with the potential to do things differently. At first your unaware of the process, but after enough cycles of incarnation you begin to retain your memories, to the point where eventually you live this one particular life as a kind of god-like super being.
gpt
I had a lot more determination and drive as a little kid than I do as an adult. I would fuck with things for 8-9 hours as a kid/teenager that I'd give up on in 15 minutes as an adult. The only positive to being an adult to me is I can think in a more systemic and less scatterbrained kind of manner and can see through things long term better. I accomplish more with generally less effort, but I wish I had the same drive and determination childhood/teenage me had. 

For things like learning musical instruments and multiple languages, we often forget how hard it was back then. When I tried learning Japanese in 2001, I borrowed books and cassette tapes from the library, and there was just a few sites around teaching it. Any resources to learn it really cost money and it would take you a lot more time than today. Finding people to practice with back then would be a lot harder, less people even had the internet, video chat would be hard on dialup, etc. Anime episodes took days to download. By comparison nowadays everyone has a smartphone in their pocket with tons of free language learning apps that allow you to speak into them, write letters with your finger on the screen, foreign stuff is easy to see, and you can make language exchange friends readily.

Musical instruments, they cost money and time from your parents, and if your parents weren't willing to invest money and time into you, regardless of your desires, you're fucked. It's way easier to pacify kids and make them 'happy' with a TV and a video game console and spending $50 a month on a new game than it is to spend the same money on an instrument and cough up hundreds a month for lessons, have another place to drive you after work when they just want to relax, give you unlimited TV dinners, Coca Cola, and Hot Pockets and just have you shut up and stop bothering them. 

To me it's a question of how much agency we really had as kids then to affect ourselves. Could we really change as much as we think?
human
This is a good point. There would be practical limits to what a small child would be able to do, especially if they were staying under cover. This is why I'd be tempted to reveal my true nature by, say, successfully predicting the future. I feel like in that scenario my parents would be more inclined to listen to me (assuming they didn't just ship me off to some government institution).

In any case, I would dedicate more time to physical fitness as a kid. I remember it being so easy to do hand stands & cartwheels as a small child. I'd put more conscious effort into maintaining those sorts of abilities. I'd start lifting right around 14, rather than waiting until my mid 20s. I'd also be tempted to reveal my power level in order to get out of having to do public schooling again, because that shit sucked the first time.
gpt
I grew up being considered 'gifted' and even having GATE style stuff happen in school, but my parents ultimately never really wanted to invest in me nor really cared much about my academics and studying past elementary school. In kindergarten I had a 12th grade reading level according to standardized tests. After my parents divorced they went from kinda caring about the future to just sheer white trash survival mode with zero thought of the future. I guess my point is even when you're gifted and a child prodigy that can predict the future, it's likely nobody will give a shit, as a child you're just an accessory/toy to their lives sorta similar to a cat or dog. 

I think if we were to go back as kids, as you said physical fitness is a huge thing, and while I had less resources than I would have liked (my mom took me out of judo for lack of money) ironically I had a cheap standard barbell and plates my father got me for Christmas one year that got lost in a move and divorce. I would have treasured that like gold. In 4th grade I could clean and press 55-65lbs for reps. 
 
I think the best thing you could concentrate on as a kid is your relationships. All the childhood friends that got 'lost' or fell away, girls in your class that could have been potential romantic partners, clubs you didn't go to, people you wronged in the past, even teachers and authority figures. I think in most ways for accomplishing *stuff* being an adult at least 18-20 or so is way better as you have a hell of a lot more resources and more agency. Of course as a kid you have more energy and time. For me my biggest change from being a kid/teenager is my mindset is a lot more positive and less 'toxic' with a lot less hate. So if I could go back to teenage me with current me's mentality that would be the hugest life improvement.