Assuming a 64 bit number for activities (more than enough, I think!), a 9 character geohash[0], another 64 bits for health (HR, BP, weight, etc.), and you've got 25 bytes per second, 2.2M per day, ~789M per year, and, given 75 years life expectancy (roughly UK average), ~60G per lifetime. Probably just fits on a 64GB USB stick / SD card.

It should probably compress quite well, also.

[0] https://www.elastic.co/guide/en/elasticsearch/reference/6.8/... claims 4.8m^2 for 9 characters.