I can attempt to answer this; have worked as a generalist at two startups, ran my own startup which got acquired and have hired a bunch of generalists. in order of preference -
1/ running your own mini startup
this is different from a side hustle - you’d want to showcase your ability to run a full stack mini enterprise, so demonstrating an ability to work across various functions is key.
i ran a d2c food business for some time, and it helped me tremendously while interviewing even though it was a “side hustle”
2/ consulting projects at other early stage firms
figure out what your “spike” is, and offer early stage companies that on a part time basis
i did a couple of brand strategy and CRM building projects since GTM is something i consider a spike. the idea is to use that as a foot in the door
3/ writing, podcasting, video content
i would consider this proof of intent rather than proof of work, but it did help while we were hiring
hope this helps !