heres my thoughts
you want an "activity" rather than stretching exercises etc ?
i have had back problems since my 30's
i now swim 3 times a week , and my back feels a LOT better for it
i used to cycle, gave that up because of it, then tried running, then found it because of that i have i had to give that up too, now I swim 3 times a week, now I feel less pain at work (manual labour, so on my feet all day) and also when i had a day off , and dont get up at 5am for work, (so we are talking 8am now) my back used to hurt all day until work next day, but the swimming has now stopped this !
Just dont stop swimming, i had the flue end of December, and the fact the Pool was closed for 3 weeks due to refurbishment, meant i couldn't swim for 4 weeks, on the 4th week, my back was starting to hurt again