Exactly as Adami stated. And yup Rumborak, as a rock climber on hiatus myself it can totally be a mindful thing. The very nature of climbing is an elaborate, physically intense thing. Yet during some climbs, the draining aspect doesn't come until near the end of the days' climbing. Even the act of hiking to a rock face- as Adami said about noticing things as you walk. The more I think about it, the whole package of rock climbing can be mindful; camping, hiking, climbing & belaying, cleaning cams.
My group aide said it best once; many of us are kind of doing it already. We just do not draw attention to it, and are not fully aware of it. Despite me feeling the term mindfulness has cheapened more broad concepts, the term is used precisely for this, as in the traditional sense mindfulness is state of awareness. So just as what was already said, we can be mindful of pretty anything. Or mindfully do anything. I haven't seen any limits- so yeah, playing music. I analogize playing in the pocket, or maybe improvising, being in the moment and not caring about a mistake or anything.
That being said, traditional meditation is the more pure form of mindfulness IMO, and something that should be a goal for many. It simply focuses on breathing, which is the core thing that we to living vs. not.