Last year, at 35, I started learning to play the piano from absolute beginner level by attending private classes once per week.
The teacher mixed practice with music theory and learning Solfeggio (I'm meaning sight reading the notes from a music sheet).
Currently, and for work reasons, I spend 5 days a week overseas and I can't play the piano at all while I'm there as I don't have a permanent residence where I'd put a cheap digital piano.
My question is whether you have ideas or advice on how I could still use some time while I'm abroad to keep learning more about music theory and other useful knowledge that I could later apply to the piano in the future. Furthermore from not forgetting the knowledge I already had.
I accept any idea: books, software, websites, mobile apps, ...
If it helps, I'm from Europe so I'm learning with what I believe is called Fixed-Do.