It was a week-long silent meditation (Vipassana), focusing on "Mindful Awareness".
It was at a point in my life where I was able to do it (and I'm grateful), but I have a kid in kindergarten now, so it'd be hard to convince my wife that it's important enough that she should single-handedly manage for a week.
Overall, I think the major benefit for me, was that it bootstrapped this mental model:
* Our minds are constantly spawning processes, and "learning how to start + keep running a debug process" is an invaluable tool for gathering data.
* Once you've gathered a reasonable amount of data, you will start to spot patterns. One of the teachers at this retreat expressed it as "wisdom will arise"
I have a long way to go, but I'm grateful every day that I have this foundation.
In an ideal world, it'd be nice to do this again, to compare where I am today (IIRC, it's been ~8 years).
P.S. there were no substances included :)