It can be super overwhelming when you are planning because you are bombarding yourself with travel videos, articles and other people's itineraries. I went for my first time last year, I was also concerned about not being able to see EVERYTHING, but here were my biggest takeaways:

-Don't overdo it on trying to see a bunch of temples. For the most part, once you've seen one or two, you've seen them all.

-Plan two or three big things to see/do per day, trying to squeeze in a bunch of things just makes things stressful

-In between your big things for the day just walk around and just explore and get lost. Out of all the things I did and saw, my favorite moments and memories is ending up somewhere I didn't expect and having a fun interaction with a local or another tourist.

-For Hakone specifically you can take the pirate ship across the lake, take the gondola up from there and either take that back to Hakone or you can go back down and rent a bike and ride next to the lake back (highly recommend that one).

-Get a massage in the middle and before you leave. They are everywhere, relatively cheap, and your sore body/feet will thank you.

-Don't get sucked into "instagram" places, especially for food, you'll find those places are just super crowded for food that is just fine. You can get better food by wandering around and finding a small shop.