As far as I know, you already know the game, but I can confirm that "12 Labours of Hercules" is a classic 1 day game. It takes about 5-6 hours to complete all achievements. It's light hearted and leaves you in good mood. Maybe a bit to easy on standard difficulty, but still a recommendation from my side.
Thanks! I'm actually playing that one myself. I've gotten hung up on a level where I'm trying to get all three stars and keeping failing by mere seconds. It's frustrated me so I've put it aside. But I'm sure I could do it in a day if I wasn't being so perfectionist (and doing it so badly).
A small sidenote; both a Story About my Uncle and Gone Home are now available on GOG. You might want to update the opening post.
Yes, I was just checking the list today and it reminded me to install A Story About My Uncle in order to check it. The version on the list is the original freeware one. I want to see if the version on GOG is significantly longer. The filesize is about twice as large.