Our son has just turned 7. He's into creating CDs. This involves mixing up tracks from various existing CDs into a new mix. He then elaborately decorates the CD with textas and creates the booklet, track list with times, front & back covers & spine. It's very important for him to cram as much music as possible onto the CDR. For some reason, when he tries to create one over about 78 min in length our computer usually fails to create it sucessfully, which causes a huge amount of frustration!!
We now limit him to one CD per week. He'd much prefer to create them daily! Once it's created he rarely plays it. It's all about the process, not the outcome. He's created about 50 CDs in the last 9 months, plus a similar number of 'coasters' (duds).
It's little wonder he loves this as the process has so many of the elements that he loves: time & numbers, pop music, using the computer for serious stuff, rigid procedure & rules.
He has other passions too, but that's the most sustained one.