In regards to going back to WoW, just don't. I also played for quite some time. I started on release day and played for around two years. I quit once due to changes to classes and such, but mainly it was also realizing that it has inherent flaws to it. I went back for a bit, mainly because my wife still wanted to play ( we played together ) . Then, while on our honeymoon we both decided to quit, since we found so many other things to do.
In regards to what to do with your summer, man I'm jealous. It would be nice to be a teenager again and to have that free time again. Since it is the summer I would do as much outside activity that you can. I saw you mentioned biking, drawing, reading and other activities. Those are great. Continue to do those more. Also, yes, spend time with the girlfriend, I'm sure she is much more worthy of your time.
When there are bad rainy days, do the reading and drawing. If you do have to play a game, what about board games with family, or card games?
Then, when you do get though the summer, Spore will be out, and I would recommend that one. Why? Well, it's not an MMO, but more sim/strategy based that will engage the brain. The best part though is you can walk away and not worry about missing things. The game will be in the same spot when you left it.
Seriously, though, you're young and have time. Enjoy it. It doesn't come around again.