Things you did/upgraded that surprised you how helpful they were

Only two?

Car lift.
Holeeeee crap, what a huge improvement in convenience. No more squatting to do brakes, no more flat back transmission swaps, you can stand while doing everything.
Air tools.
Time saver for sure but also a great help when in tight spaces where a ratchet may only be turned 1/8 of a turn at a time.
Supportive seats.
We all have some experience with suspension improvements but for a cornering guy, stock seats are inadequate. A good seat with side bolstering makes a huge difference.
Overdrive transmissions.
Yes...This is another issue where using some later model tech helps a classic be more comfortable and livable for distance trips. My Tremec 5 speed reduced top gear rpms by a third...Fifth gear is a .64 ratio. 2000 rpms at 70 instead of 3000 is a gigantic improvement.