Small backpack 25L and a waist pack. Buy slide sandals or flip flops from your own country. Big plastic bag for the sandals. Wear breathable training/running shoes and bring three pairs breathable thin socks for travel. 2 t-shirts. 2 lightweight shorts. Leave room in backpack for 100฿ elephant long pants for temple visits - buy those here in TH.
Passport in clear sandwich plastic bag. Debit card (the charge is 220฿ per transaction - use the yellow kungsri ATMs because you can get 30,000฿ per transaction, and remember to hit the button bottom left “NO conversion”.
Keep total below 7kg. Backpack and waistpack. Then you can fly around Thailand with no extra baggage charges.
Nokair.com is one of the cheap airlines.
Bring an extra debit/credit card or two.
Bring an extra bin bag to put your stuff in in case of rain.
Agoda.com and booking.com to book hotels/hostels in advance.
Grab and Bolt apps for taxis.
Airport train from Suvarnabhumi Airport basement (Bangkok) into city, or direct bus to Pattaya/Hua Hin from the airport.