Hi Rose,
Glad you enjoyed the horse ass! In terms of canyoning or rafting, you should check out Minakami Canyons in Gunma, a couple of hours north of Tokyo. As a rule, Spring is rafting season, while Autumn is canyoning season.
I can’t give personal advice on tattoo friendly onsen as I don’t have any. However, we do have an onsen and sento in Tokyo. If you look at the tables for each place you can see if tattoos are allowed.