I will second (or third) a couple of games on here.
1) Oblivion - I played it on the 360, but this was an awesome game. Put well over 100 hours into it, really none of the time felt wasted with the option to quick travel. Though, I'll admit, I really liked exploring the countryside finding caves, etc. Overall, if you like role playing games, I would think you'd enjoy this one.
2) Little Big Planet. I got the GOTY edition packed with my PS3. It's still an expensive game ($50 or so), and the actual main game itself is rather short. That being said, there are tons of great user created levels. I was scared that they'd be a few good ones and mostly crap, but in my searching, I find good one after good one. I'm sure some out there suck, but it's not too hard to find some really great levels. I believe there is over 1.7 million user created levels out there.
And, sure, you can create your own levels. I think it's worth a few hours playing around in the level creator just to see it work. However, I think you have to have an artistic mind and quite a bit of time to make a good level that others would want to play. But, this game can easily be worth it even if you never touch the level editor yourself.
