Another older title I don't see mentioned is 'Eufloria' (I've only played the classic version not the HD one) - Basically just plant trees, migrate spores to other planets, rinse and repeat.... Very easy to play for 10 hours without realising.
I was indignant because I had already mentioned this. Except I see that I haven't in this thread. It was quite recent, so there must be another 'relaxing games' thread floating around here somewhere.