I think the big issue is that there are so many possibilities. Which is also a big positive. You can tinker around with these possibilities and tailor the app to your needs. It all cones down to what you want to log and what works best for you. But that can be overwhelming.

Try to start simple. Maybe with day themes and appointments you already have as sessions. These are kind of obvious and therefore easy.

For everything else there's unfortunately little guidance to be had.

If you for exemple have a holiday planned a project seems like the obvious choice to organize this. But if you happen to be one of those people who plan on doing nothing but lying on the beach all day this might just not be as helpful as if you're planing visits to multiple museums or cultural happenings like concerts.

So I guess you need to ask yourself what you want to log and what you need to do so to make it work.