- embodied

- self-directed

- open-ended

and includes things like playing in a band, performing for others

Doing crossword puzzles is:

- all in the brain

- exclusively solitary

- closed-off (just looking for the right answer)

Not an expert, but I think you are more likely to escape dementia if you balance physical, mental, and social activities, as well as do things that are opened-ended (let some light in!)

Crosswords are too simple, they only teach you how to get better at crosswords. Learning an instrument is a whole lifestyle - you need to combine aural, verbal, proprioception, social, and so on.

While crosswords help with exercising pattern recognition and recall, you're not generally learning anything 'new' (except the occasional word, and this diminishes over time).

Sight-reading random piano sheets, for example, would exercise most if not all of your faculties