When I was a boy, I used to lie on the grass and gaze at the white clouds.
Literal
I [topic-は] boy [possessive-の] time often grass [possessive-の] top [on-に] lay-down-[sequential-て] white clouds [object-を] gazed [nostalgic-past-ものだった].
A vivid nostalgic scene. 寝転ぶ ('to lie down, sprawl') in て-form chains to 眺める ('to gaze at, contemplate'). 草の上に = 'on the grass.' The ~ものだった ending bathes the entire recollection in warm, looking-back glow.