I think hour building gives you an excuse to do some fun flying to interesting places that would perhaps be hard to justify if you weren't hour building. I don't think cost should be the limiting factor. Flying to France, Spain etc. will be something you're never likely to forget and you will automatically be gaining experience and learning new things by undertaking such flights.
I think you should try to do new things and challenge yourself. Don't be one of those fools that spends an hour a week in the circuit! Try to fly something with a wobbly prop and retractable gear, something on floats, something aerobatic, and maybe try some gliding. (I think you can include upto 30 hours of gliding in your hour building)
I also think it would be a good idea to save your last 10 hours or so for mentored hour building with guidance from a CPL instructor to make sure you're not going to struggle on the CPL course.
Most importantly have fun with it! You'll never get the opportunity again.
