I'd also say that you should get the other two GTAs.
If you're looking for adventure/platform games, I can't recommend the Ratchet & Clank series enough. The Jak series, too.
I also have to mention ICO, which is very cheap everywhere now. It's a somewhat short game without too much in the way of gameplay, but the experience is akin to what "games as art" might be.
And of course, the obligatory, "wacky Japanese" recommendation for Katamari Damacy.
Metal Gear Solid 3 is also a great new game to pick up so long as you're willing to sit through some half-hour CODEC conversations.
I'm not sure if you already own these on other systems, but you should also give a try to Burnout 3, SSX 3, and THUG2 (though I personally still prefer THPS4 in the entire series). The thing about those 3 games is that they're so good they even appeal to people who don't care at all about racing, snowboarding, and skating.