If you enjoy myst-likes I'd also recommend Quern. Very good.

my 6 year old nephew loves myst and riven, I suspect he does not really understand the puzzles but likes the atmosphere and spending time with me as we talk about it.

Based on this I picked up Obduction to play with him and he loved it, still I suspect not really understanding the puzzles. heck, I loved it, I had forgotten how much I enjoy cyans brand of puzzles(Apparently I have a real soft spot for puzzles based on strange base number systems). I had to keep stopping myself from playing when he was not around so we could enjoy it together.