If you need to escape the city properly, take RER B to parc de sceaux station. Huge park with the formal french garden section everyone photographs, but also kilometers of actual forest trails most people never see.

Château and gardens are nice but head past them into the woods for proper walking trails. There's a 5km loop that feels like you're in the countryside, not 30 minutes from central paris. Way better than the tiny jardin du luxembourg if you actually want to walk and breathe.