First thing I'd do is test it. If I nick my arm, is it healed the next day? Does it happen at a precise time and will I waken at the same time? Will the people I don't interact with always do the same thing? If I eat different things, will it impact my weight or poop for the next day? Will I continue to develop more muscle? Will other people physically change? What will happen to births and deaths?
Second thing I'd do is acquire skills, similar to how he learned the piano. Learn musical instruments, languages, chess, health, school topics,etc. For a normal lifespan those skills aren't too valuable, but they can be very valuable if you get free years to learn.
Third thing I'd do is entertainment. Mix of reading, spending time with friends, driving around, etc.
Ideally I'd be able to repeat a longer time period. Say, a week or month or year.
This post was edited by carteblanche on Apr 27 2016 02:11pm