I'd be interested to hear what are people's most useful customisation options within ArcGIS Hub that they're taking advantage of.
For those that don't come from a HTML / CSS coding background absorbing some of this knowledge could be quite useful.
I'll kick things off with the Shaped Card which has been really useful to show off applications, different from the traditional Gallery in the Layout view. You can find a full link to these here - Calcite Bootstrap. Further down the page you can find some other customisation options for statistics you may be interested in.
Looking forward to hearing from everyone.
To add on to this, there's a whole page dedicated to Hub site customization:
I've only really played around with these some myself but I've sent this page along to several users who have said it was a super helpful starting point.
Here is a comprehensive presentation about customizing Hub sites (and Enterprise Sites). It covers topics & tricks on each card and the bootstrap 3 CSS system.