Besides a decent socket set (with deep sockets!) and a roll-up (compact) fabric bundle of SAE and metric combo wrenches, one of the most often used little tool sets on our boat is a little plastic pouch of "ignition wrenches".
They are thin, open on both end, and one end is offset to a side. Darned Handy for a variety of jobs where it's difficult to get at a nut on the back side of something.
Another tool that's useful on most every job, are a pair of ratcheting right-angle screw drivers -- straight and phillips.
They are short-handled and can get at a lot of fastenings that are impossible to reach with a straight-handle screw driver.
Further from wrenching.... vital tool is a hammer! For the boat it's a stubby handle 2# "drilling hammer" that I picked up cheap from Harbor Freight. Lots of jobs require some force in places where you can only move the hammer about an inch, but still need some torque delivered when it strikes....
That, and a cheap 1" chisel that gets reground as needed.
And... used regularly for about 20 seasons, a
Fiskars hand crank drill. No cord or battery, no problem.
After three decades, it's amazing what the tool box finally starts to boil down to!
Also, all that useful stuff that Claude mentioned, too.