Personally, I make chains in some junk metal to level up metalcrafting. They return the whole bar upon melting, so nothing is lost, and the masterworks can be traded away at a decent value.
Once, with vein after vein of galena and the usual superfluity of lead, I trained one up making lead crafts. Then I trained up a gem-cutter by cutting junk stone, and a gem-setter by encrusting them in all the lead crafts. It took a few years for them all to be legendary, but with the right stockpile links I kept them all on repeat and it went on without any management at all.
My current project is to dig a shaft down from the surface to light up a portion of my farms, in order to grow surface plants. My fortress sits between the third cavern and the magma sea: there were no good solid chunks in the caverns to dig through, so now I have to wall in those layers of the shaft while avoiding dangerous critters and that one forgotten beast.
I'm not sure which will be more tricky -- the security itself, or the actual logistics of building the walls and digging down into them without cave-ins. Also, I've noticed that miners can climb a few z-levels to get out of a shaft for a drink or a meal, but won't climb back in to resume the work. It's hardly a megaproject, but it's very fiddly!