A few decades ago, I took several trips to Thailand only booking some of my accommodations in advance. With the growth of tourism, now that I know where I want to go, I prefer spending time looking for a good quality-price ratio based on my criteria and booking with cancellation options on platforms A or B, rather than searching upon arrival. On vacation, I don’t have time to waste

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A few years ago, I was showing the country to some friends, and the South was severely flooded while I was in Isan—railways, roads, airports, phones, everything was cut off. I had to change my plans (Koh Sukorn, Pattalung, and Prachuap Kiri Khan) to head north, but my favorite little hotels in Chiang Mai and Chiang Rai were all fully booked. I found places, of course, but they were less pretty, less friendly, and less well-located.
Especially with a child, I’d recommend booking in advance.
I don’t know when your trip is planned, but when it’s very hot, kids love having a pool in Bangkok, for example. On the other hand, there’s no need to pay for a pool in the north in February—it’s too cold.
If you know where you’re going and what type of accommodation you want, you’ll find people on this forum happy to give you great tips.
Have a great trip.