I am about to embark on a 1.5 month sabbatical - not quite as long as you (I'm jealous!), but here's how I prepared it - perhaps it'll give you ideas.
Over the past few months, I've been keeping a list of things I'd want to focus on. Ultimately, I decided to trim this list to 5 things, because I don't think one can focus on more than 5 things at once. Here they are for me (unranked, equal importance to all):
- Working on my Japanese (I'll be in Japan)
- film photography
- sketching/drawing/painting
- reading
- programming
For the last two items, I've been keeping a list of books to read, and topics to investigate/projects to work on over the past few months as well. Then I trimmed theses lists to end with 1 book per week to read and 1 programming project per week to work on.
Of course, I'm likely to deviate from it, but having these lists and priorities as my guiding star will be helpful.
If I were to do this for 6 months, instead of 1.5 months, I'd probably break it down in 1 month chunks, with varying priorities per month, and a bit of loose time in between said chunks to account for unexpected things/rabbitholes to go down.
I also have a 5 day mountain hiking trip in the middle of it all, because in my experience there's nothing better for your brain than disconnecting absolutely from reality, and centering your days only around walking/eating/sleeping/looking at beautiful landscapes.