The mental health benefits of regular movement are well established, but the exact type and intensity of exercise matter less than most people assume - consistency is the factor that actually drives the benefit.
A consistent moderate routine - regular walks, light activity most days - tends to support mood more reliably than infrequent intense workouts separated by long gaps of inactivity.
Exercise done outdoors, even the same activity done indoors, correlates with additional mood benefits beyond the movement itself - likely from a combination of natural light and environment change.
An activity genuinely enjoyed is far more likely to become a sustained habit than one chosen purely for its exercise value - the mental health benefit depends on consistency, and consistency depends on the activity being something you'll actually keep doing.