HVAC. Opens you up to a lot of the other trades. You'll learn plumbing, electrical, welding and can even branch off into specialty trades like building automation.
Not everyone can wrap their head around the technical parts of it though. I've had guys go to school, apprentice, more school, thrown into the fire... and still not get that good at it. Takes a certain kind of mind for the diagnostics aspect of it... but doing the installation side isn't usually quite as technical and can often be a little better in many respects.
Almost any tech school has night courses to get a certification, federally you only need an EPA license... which isn't a real hard test. States have their own licensing requirements... but you'll find that many don't require licensing at all unlike pretty much every other trade.
6 figures isn't too hard to hit, but a lot of that can be OT money unless you get into a union. I know of unionized companies near me that pay over $60/hr. I know of non union ones that pay $40 for experience and $30 for guys right out of trade school.