I keep coming back to this game, occasionally playing a few levels. I'm now more than halfway, but there are still many to go (500 levels in total!). Despite its unassuming appearance and simple progression mechanics (solve a puzzle -> unlock the next one), this might just be one of the best puzzle games of the huge bundle it was in.
All of the puzzles are solvable; you're never truly stuck for a long time. Nevertheless, they all require some thought and it never gets trivial. If you're searching for the perfect relaxing puzzle game to get you into a flow state, this is it.