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Well, I got an idea for quite the late game to add happiness. A circus troupe, as the title suggests.

After you have 1) an X population, 2) a port, 3) a town square with no festival going on, a troupe could visit you in a ship, landing in a port and pathfinding (like an army squadron would) to the mentioned town square. Then, they ~~could perform a fancy animation~~ would put their tents and give you a festival-like happiness boost for a year, for free. But this would provide you with no new peasants, to balance it out.
In addition, you could talk to them and ofer them your "patronnage"; paying them gold (on library level), they would stay, unlocking a new building (a circus).
After you've built it, it would give you a ranged happiness buff, as usual, but also a slight global happiness buff. You would have to feed them, but possibly the benefits would be greater