I’m playing Syracuse and I’m swimming in PI. Maxed it out at 500 and jammed the stability button just to do something with it.
But thanks for the suggestions. Looks like I’m going to have to deepen my knowledge of the game because I don’t even know what half those things are! For example I had no clue I could found a city and have no idea why I would do so.
The province improvements are super pricey. Like $80 a pop. Not cheap at all.
Spend PI on Divine Sacrifice (stability), that's ~45-50 PI every 5 years. Always nice to have that first divine sacrifice running (you can spend more PI on additional sacrifices to get a bigger benefit but I don't think it's worth it).
As others have said, other options that are usually good investments include provincial improvements and upgrading settlements to cities and cities to metropolises as well as dedicating holy sites (best done on larger cities).
You can also spend PI on changing laws or your pantheon, or diplomatic stance, or policies, etc.
As for why you would want to found cities:
More cities means more population that can be supported, and more building spots, and increased civilization level in the territory. All of that translates into
more money, more manpower, more tech, and more trade routes (which then leads to more money, manpower, and tech). It used to be that you didn't want to have more than 1 city per province, but changes a few patches ago made it more balanced to where additional cities aren't so poor an investment. Basically, you'll want as high a population in a province as your food generation allows, which means you want at least one city per province (but more can be good) but just make sure you don't build cities on top of food resources and remember that there is limited benefits from building additional cities when there is more room in your already existing cities.