The most laid-back or relaxing Zeppelin song would be "All My Love". I think it's an an obvious choice, and I'm surprised more people didn't choose it. I was going to pick "The Rain Song", but then I remembered that brief section where the band crashes in before returning to being mellow. "Going to California" would be a close 2nd place because of it's late '60's sounding, hippie-folk vibe. I can't count an instrumental like "Bron-Yr-Aur" because it's not truly a song. And, for me, "Down By the Seaside" is simply corny. Next to "Fool in the Rain", it's my least liked Zeppelin track.