Oh, I know — while I'm at it, why not take a stroll to the forest road for a little forest bathing...
Literal
That's-right, if-anyway walk [incidentally-がてら] [to-に], forest-road [to-に] going mini forest-bathing [or-something-でも]...
そうだ as a sudden-idea marker ('oh right, I know!'). どうせなら ('if you're going to anyway') sets up an opportunistic suggestion. がてら is a conjunction meaning 'while also doing / incidentally as' — here 散歩がてら means 'taking a walk as the occasion.' プチ is the French-origin prefix for 'small/mini.' 森林浴 ('forest bathing') is shinrin-yoku, the Japanese wellness practice of immersing oneself in a forest atmosphere. The trailing でも + ellipsis leaves the thought casually incomplete.