Gardens are great (mostly for fountain of youth) but I want to say a nice few words about the Great Harbor, and passive percentage buffs for the economic min-maxer
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Firstly, I'm playing a Merchant Republic (Pisa) and I've put my GH in Messina, which I own - I've put it there because if I put it in Pisa instead I'd be losing it in the election roundabout (as I do NOT want to constantly challenge (I'll explain why)), whereas putting it in Messina I still get the passive buffs, even if I don't get the official ownership. I have also built two barony level cities in Messina - San Marco d'Alunzio & Troina I think they are, both of which I hold in my demesne personally (for those of you who don't use minor baronies, let me teach you the ways of the passive buff). I also have my republic special holding in Messina whenever I am not the doge. Sadly the "A Town Is Born" event has not fired yet, but I still have two features to build on it. When it triggers I will build another city and I will have 5 holdings in one county, all having buffs applied to them.
It's all about city tax. The tax buffs are all added in the first half of buffing (before adding stewardship). Each stage gives you 2.5%, road tolls +5%, Lighthouse +5%, Harbor office +10%, piers should add +2.5% according to the wiki as well but for some reason my game says 0.25% - regardless that's a +30% tax increase per city. In addition you also get 130 trade value as well on the province, plus another 30 for the wonky piers. Remember this is being applied potentially to 5 different holdings.
Where my other cities make on average 110 gold a year (at a ruler stewardship of 15+), these cities rake in the mid 140's and my republic holding jumps to the 150's/160's from the 110's/120's, in fact these make enough money (+150's/160's) to account for more than an additional city, without the need to hold such. I'm not especially adding on the passive buffs you get when the county is prospering ofc, because that is applied in other counties as well, and I'm not even touching the option to send your steward to increase tax in a county (although it would increase your income dramatically). Regardless, this makes you an economic powerhouse.
EDIT: Just done the numbers, and it's about 15/16,000 gold a century extra on what you would have already made (assuming you keep control of the county and keep raiders away), and well over 50,000 gold per century (for just one county)
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Yeah, it's no fountain of youth, but as a niche MR min-maxer, I do like the Great Harbor