We are thinking of travelling to Thailand this summer with our two teenagers (16 and 13) and would be grateful for some recommendations on resorts/areas to stay. We weren't planning on visiting Bangkok and wanted a beach resort. Also what is the weather like as regards rainfall? Is it solid rain all day or more like quick tropical downpours? Many thanks

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Many long distant flights first land at Bangkok. Bangkok is an interesting city to visit and can be cheaper than many good beach locations. Bangkok also has summer - like weather just like beach areas. If you first fly to that city and transfer to a local flight on your return don't try to schedule a last minute flight to avoid Bangkok. Your connecting flight could be cancelled, could be late or whatever and you miss your ride home. Missing your flight home could cost a lot of money for buying new flights. (Millions and millions of family groups have visited Bangkok over the years.)

Bangkok has 2 airports. Know which one your departing flight leaves from. Both airports are not that close together.

What beach you choose will be influenced by normal weather patterns for when you visit. Do some research for Koh Samui. Link below:
Koh Samui in July: Weather, What to Expect and 15 July FAQs

Even during the rainy season tourists still visit Thailand every single day. Many tourists are repeat visitors and choose low season to save on hotel prices. Google search will give you information for various beach locations for your "summer visit." For some places in Thailand you may get heavy rainfall for a few hours then it stops. Some areas not much rain. I like the rainy season because the trees and plants look alive. Thailand might be bigger than you think. You could get rain in one location and be bone dry in another city.

You have a lot of time to work things out. Right now if you haven't got passports get them! For changing your money to Thai baht do it in Thailand for best return. There are some bank money changers at the airport where you can change a little money for first night. Rates away from the airport might be a little better. But usually for Western currencies you get a better rate in Thailand. Rates are not fixed. For me hotels and stores have a worse rate than the places on this link.
https://daytodaydata.net/

For any trip to a foreign country always Google up scams you might encounter. Renting water craft is one scam to look over. (You get charged for damage you did not notice when you took over. Look over any watercraft you rent and bring the damage to the rental agent and photograph it.)

A visit to Thailand is not a weekend trip if coming from a distant country. If from Europe or N. America my advice is to stay at least 10 to 14 days.

Good luck.