This has been on my mind too. I ride an indoor exercise bike on rainy days or in the winter. We have tons of trails around here. I want to get a mountain or trail bike, but that's another rabbit hole and hundreds and hundreds of dollars I don't want to spend.
Discussion
Just walk wearing minimalist shoes/sandals (or barefoot if you can handle it).
Your body needs to feel the impact of walking to stimulate bone formation.
Cycling makes for weak bones.
Walk in the sun like your ancestors.
https://www.bicycling.com/health-nutrition/a26100802/cycling-bone-risk-strength-training/