while out on a walk; while strolling
Adverbial phrase meaning doing something while taking a walk, often implying the walk is a secondary or incidental activity.
While out on a walk, I stopped by the neighborhood bakery.
I like taking photos in the park while strolling.
Compound of 散歩 (さんぽ, 'walk') + がてら (suffix meaning 'while doing something'). The suffix がてら derives from classical Japanese, originally meaning 'in the course of'.