Is it better to take the train or fly?

Fly for long distances (e.g., North to South). Take the train only if you want the 'experience' of a sleeper berth or are traveling between Bangkok and Chiang Mai.

Do I need to carry cash everywhere?

Yes. While malls and big hotels take cards, street food vendors, local taxis, and small ferry operators are cash-only. Always keep 2,000 THB on you.

Planning a Three-Week Trip Through Thailand: Bangkok, Islands, and the North

Divide your three weeks into three distinct segments: four days in Bangkok, seven days in the northern mountains of Chiang Mai and Chiang Rai, and ten days island-hopping in the south. Use domestic budget airlines like AirAsia to jump between these regions to save time and maximize your experience.

Map your flight route. Fly into Bangkok (BKK) and plan to fly out of Phuket (HKT) or Krabi (KBV) to avoid back-tracking. Book your internal flights at least 4 weeks in advance to keep costs under $60 per flight.
Secure your accommodation strategy. Book 'flexible' cancellation rates for your first two nights in each location. Once you arrive, use local apps like Grab or ask your host for recommendations if you decide to extend your stay in a specific spot.
Coordinate your regional transport. Use the overnight train from Bangkok to Chiang Mai to save on one night's hotel cost. In the islands, use the 12Go Asia website to book your ferry transfers between islands at least 48 hours in advance.

Is it better to take the train or fly?: Fly for long distances (e.g., North to South). Take the train only if you want the 'experience' of a sleeper berth or are traveling between Bangkok and Chiang Mai.
Do I need to carry cash everywhere?: Yes. While malls and big hotels take cards, street food vendors, local taxis, and small ferry operators are cash-only. Always keep 2,000 THB on you.