I first read your response shortly after you posted it. The Isaiah passage caught my eye and I wanted to reflect on it a bit before responding. I must confess that I'm struggling a bit to put this passage into context. The way that it's translated in your post above, as well as in the ESV, the words "finding thine own pleasure" seem to stand out. On the face of it, it would seem that God is saying that fun and the Sabbath are mutually exclusive. And I know many Christians also feel exactly this way. Just recently I saw a picture of a sign at a Christian recreational facility asking people to refrain from using the ballfields on Sunday. This too is the world that I grew up in.

Digging further into the Isaiah passage, the footnotes in the ESV say that the word "pleasure" can also be translated as "business". But business is such a broad term that it's tough to think of it exclusively as the work that we do to get paid. Perhaps something can be gleaned from the same chapter, verse 3b, "Behold, in the day of your fast you seek your own pleasure, and oppress all your workers.". In the verses leading up to those you cited, it feels as though there is a connection between a day of fasting and the Sabbath. Or maybe I'm just reading something that is not there.