- Samorost 1 and Samorost 2. Pretty and imaginative. They probably take a half-hour each to complete. Relaxing.
- Grow. You might get this in ten minutes, or in a couple of hours, depending if you're lucky or unlucky. The components interact in interesting ways.
- Takagism.net has links to Crimson Room and Viridian Room. They might take an hour or so each, longer if you get stuck on something stupid.
- MOTAS - the Mystery Of Time And Space. Not so much art as obsession. Don't start this unless you're unemployed and you have serious time to waste.
I'm sure a search engine will turn up cheats for any of these if you get stuck.