There's a lot out there about various exercises or games you can do to keep your brain sharp.
I would like to compile a list of things for sharpening your mind that I think are interesting, which you can do at little or no financial cost.
I think that music practice can take many forms, many of which are not only intellectually stimulating, but also possibly quite meditative: a way to let go of worries or agitation.
Here are a few sites where you can do some rhythm practice (i.e. reading rhythms or imitating them). The difficulty is quite variable, from beginner to advanced: