Five decisions.
Start here.

For food, temples and city wandering, build around Bangkok and Chiang Mai: Yaowarat and Wat Pho in Bangkok, then the old city and Wat Phra That Doi Suthep in Chiang Mai. For a city-and-beach trip, pair Bangkok with Phuket. Phuket Old Town gives you shophouses, local food and a Sunday market; the west-coast beaches give you a different base.

For a Gulf island trip, start with Koh Samui and add Koh Phangan or Koh Tao only if you want the extra crossing. Lomprayah links these islands, but each hotel change adds a journey from your room to the pier and another from the next pier to your room. With a week available, our starting suggestion is two main bases. With two weeks, three can work. Those are suggested allocations, not minimum stays.

For ten nights with Bangkok flights, try three nights in Bangkok, six in Phuket and one back in Bangkok. That gives you two main stays and puts you in the departure city before the international flight. Compare that with adding Chiang Mai: the extra destination requires another flight and reduces the time available at the coast.

If a sleeper train is part of the appeal, count the night on the train separately from hotel nights. Search Bangkok–Chiang Mai using Krung Thep Aphiwat in SRT's D-Ticket system, then check whether the selected accommodation is a seat or a berth. Do the transport search before paying for non-refundable rooms at both ends.

If Samui is your destination, compare a Bangkok Airways flight to Samui airport (USM) with a flight to Surat Thani (URT) plus a road-and-boat transfer. Lomprayah sells transfers from Surat Thani Airport to the Gulf islands. Put the airport transfer, ferry and journey from the arrival pier to your hotel beside the flight fare before choosing.

Do the same with the accommodation: compare a room near the beach or food area you intend to use with a cheaper room that needs repeated rides. For a group, divide room and vehicle quotes by the people sharing them, but keep individual flight and ferry tickets per person. This usually reveals more than trying to set one daily spending figure for the whole country.

Use the Ministry of Foreign Affairs' Thai e-Visa site to check the visa information relevant to your nationality, purpose of travel and place of application. Do this before committing to a stay length. Visa arrangements and the Thailand Digital Arrival Card are separate processes.

The official TDAC service is free and the card is not a visa. Its guide asks for passport, personal, travel, accommodation and health-declaration information. Keep your first accommodation address and flight details ready, then follow the portal's current submission instructions near arrival. Save the confirmation privately with your travel documents.