We had another meeting on the 5th of February to do more brainstorming (and have a good meal). The following games and libraries were mentioned, and are linked here after the jump. Eitan transcribed them and emailed them out, and Philip formatted it for the blog.
Fl0w
A game that allows the user to control the difficulty level that he experiences.
Cloud
A game designed to promote relaxation (you can read more about it). Downloadable only.
Hell of Sand
A strange, addicting toolkit. May have trouble loading on some browsers.
Pontifex II
We didn't talk about this, but I wanted to add it anyway. It's a bridge building construction set. Downloadable demo only. If you want, I can get you a copy of the full game.
Defend your Castle
A good example of how simple gameplay, executed nicely. Enjoy.
Incredible Machine
Rube Goldberg toolkit game. Downloadable only. I have not tried downloading it, so I can't speak the reliability/authenticity of the site.
Kitten Cannon
Classic. Enjoy.
Pachinko
I don't know of a pachinko game, but that doesn't mean one doesn't exist. I would suggest doing a search to see what's out there. You can search YouTube to see videos of the machines in real life. If anyone has ever been to the Pacific Science Center in Seattle, they have a big one downstairs (no betting involved, used only to demonstrate probability).
Here's a better video of the American version of pachinko, plinko
Box2D
Open source physics library
The full complexity and sensory barrage that you get in a Japanese pachinko parlor is more complex than the basic "balls fall down" mechanic, and that plays a big part in the appeal of pachinko.
When designing casual games, the bells-and-whistles surrounding the play can be just as important as the core game mechanic, so have a look at this video clip, which has a close-up of one of the machines and gives you an idea of the cacophony of pachinko.
One thing that a lot of these games have in common is procedurally generated levels. In terms of bang for the buck, you'll get a lot of legs out of a game whose mechanics get progressively harder and force end game by pushing the player up against the limits of their skill, such as Tetris. Players are compelled to play again and again to both improve their abilities as well play the odds of getting a slightly better level layout.
I would, however, also point to the counter example like Cloud which provides a very compact experience that it delivers on with high production values. While the experience is noticeably finite, there is a memorability created by the potency of the experience. What's particularly commendable about Cloud is that it uses more than just mechanics to establish its ambiance - the pan over stills cinematics and new age music set the stage for the simple glide and drag mechanics that force you to take it slow.
Be sure to know what you're trying to accomplish when you start planning your project. Neither of these approaches are "right," nor do they represent the full suite of options, but they can offer insight into how you plan out your level design.