Now, strictly speaking, my taste in games for Mac skews a little older, but OS 9 can handle them all absolutely fine. Gems such as Sim City 2000 are worth playing just about wherever you can get them, but the version they sell on GOG is the DOS version, and Mac got a far superior version.
I can't say the same for Civilization 2, as Microprose had really pivoted to Windows as their primary operating system, but still, it's a worthwhile game to get lost in. As is Civilization 1, the Mac port is actually excellent, even if the game seems simple by today's standards.
And speaking of something a little more ambitious, there's always Europa Universalis II. It's a predecessor to one of the games that effectively kept me away from anything past Civilization 4. Crusader Kings 1 also got a PPC port, but it's OSX only, and really, I have no idea how I played that game in the first place. EU2 seems like it should at least be playable. But then, at the time, Paradox games were sort of known to be buggy, so who knows?
Of course, I understand, you want Mac focused games, then look no further than the Escape Velocity series. Only one of them ever got a Windows release, this series of 2d Elite-likes is yet another to easily lose hours in as you go from a guy in a shuttle to something so much more.
Love a game where you only start to understand how to play after a hundred hours.
