I spend most of my time making imperfect tools that I could buy cheaper. It's just so satisfying to build something yourself from a design though.
Being able to take soft silver-steel, machine a point, then harden it with a blowtorch is just magic.
Even just staring at commercially made parts and reverse engineering how/if I could make it on my little hobby lathe with the bar stock I have is really fun.
My favourite projects have been small custom parts for my bikes. Nothing is perfect but it all works and you can save loads of money[1].
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1. Such lies! I haven't saved any money because it's always "if I just had this 200 quid tool I could make this one time 20 quid part". Best to acknowledge it's just a fun hobby. :D