I’ll second adjusting seats. For some reason, whether it’s an old or new style clamp I always have to try three times to get it leveled. Also getting old style county brake pads just right. Every time I think I’ve got it, I find the total for the angle or something isn’t quite right.
On the other hand, adjusting derailleur cable tension just right is really satisfying. The feel of a smooth repacked bearing, whether it be a bottom bracket for wheel, is also really enjoyable.