The Chao Phraya Express Boat is Bangkok's riverine public transit — a working commuter ferry that doubles as a front-row seat to the city. Fifteen baht. No air-con. Absolutely worth it.
Bangkok's river predates every road, train, and highway in the city, and the express boat is still how a huge chunk of the city moves. It's chaotic, it's loud, and it costs almost nothing — and that's entirely the point. The real decision isn't whether to ride it; it's which flag to board.
Completely — it's one of Bangkok's best-kept secrets, a genuine way to reach temples, markets, and riverside neighborhoods while skipping the city's notorious traffic entirely.
Not free, but close: the orange flag line runs around 14 baht per ride — prices move, so check before you go, but it's deliberately cheap public transit.
Board the orange flag line at Sathorn (Taksin) pier and hop off at temples and markets as you go north; ignore any pier staff pushing the tourist boat and telling you the orange line isn't coming.