Few things have satisfied me as much as becoming obsessed with a project or problem and making something come out of it. I found the grad student life hard to beat. Unless your advisor is very controlling, you've got flexibility on many fronts after you pass qualifiers. You can think about problems that excite you. You can work the hours that suit you best, wearing the same clothes you wore all through college.
Vacation time is flexible.
This depends a lot on the program you are in. The faculty told us this, and I didn't really believe it. Now, I'm kicking myself. At the very least, take abstract algebra. Take as much upper level math as you can.