Is there a way to exit Power Armor before it runs out of power? There are things you can't do like sleep, wait, use workbenches... Is it possible to step out of the armor to do such things while it still has energy? Or do you just have to wait until the current fusion core is depleted? I haven't found a way but I really hope I'm just missing something...

Just hold action button at any time and you get out of the armour. You don't have to wait for the core to drain.

You may want to also get in the habit of removing the fusion core from it if you plan on leaving it unattended for some time. Npcs have been known to take them