Once you have your Mac, you'll have access to the Mac App Store (it's installed with the operating system). There are several adventure games available there. Age of Enigma Special Edition (has an extra puzzle that's not in the PC version), Jolly Rover, MacGuffin's Curse, Botanicula, and the Secret of Monkey Island and Monkey Island 2 Special Editions are some that come to mind.
Also, most of Telltale's games are available for Mac, and if you buy through their website
you get access to both the PC and the Mac downloads.AGDI
's King's Quest and Quest for Glory remakes are available for Mac (and they're free downloads!)
Adventure Gamers is tagging games by platform now. If you look here
you'll see recent articles about games available for (or coming to) Mac. And you can also search
by platform and release year to find recent ones.