на спине yes, If I had known that one, I'd have used it! Makes perfect sense.

Как-то удивленно смотря на свои кроссовки, которые шли внизу, Саша думал об этом. I realise they don't tend to move on their own, I was trying to express the idea that, from his point of view (tired boy) they seemed to be moving without him making them move. Difficult idea to get across in my own language!

увидел вокруг себя that's pretty much literally how we would say this in English, so I just translated literally, as a guess.

The gist of the story (its a bit compliocated) is that the mother and son go into a shop, and while looking at the rabbit, they hear the voice of the shop girl, who they can't see, so they think that the rabbit is talking...