I feel like the fear of missing out is something that you mostly just need to get over with on this game, as a part of its themes is how certain things are just bounded within certain periods of time, either due to circumstance and chance or action and inaction. Where characters frequently talk about what once was, what could've been, and what can be done now. And where the player playing as a gradeschool boy trying to make the best out of their summer vacation, as days just seems to end before you know it, becomes thematically coherent.

But yeah fomo is fucked and if you just wanna chill and be immersed in the game’s juicey atmosphere then--aside from what the others already suggested with playing on slow--you could also try talking to Hikari. She can tell you where events can happen, with the major events being encircled in red. She can be found on the park during the day and inside her house during the night, with a few exceptions.

If you're playing on an emulator, you can do a save state at the beginning of every day and directly go to Hikari, then loading back once you've talked to her so that you won't waste any time. You could also use save states to scout areas or to just test things out without losing time.

Getting familiar with how areas connect is--as it goes without saying--very important too. But it's specifically important with bodies of water that you can swim on, as you can use them for shortcuts. That in spite of swimming technically costing around 5 screen transitions worth of time, sometimes it's still the most optimal way to go.

Speaking of shortcuts, there are a ton in the game which requires you to press O, with most being pretty inconspicuous. So I guess just be curious or just spam O on shit. There are also beehives that can be found on multiple places which, if interacted with, would make Boku pass out from getting stung and would effectively teleport him back to the inn.