Juggling is such an underrated activity. Not only is it rhymthmical which, like walking, is quite soothing for the body, but it also promotes cross-hemisphere communication in the brain.

Our hands get a lot of neural real-estate (see grotesque homunculi) so, unlike walking, activities targeting their communication have significant potential for shaping neural patterns.

Apparently Einstein had an above-normal number of connections in the corpus callosum. I reckon he was an ambitious juggler.