Take what you are comfortable with or already have. No need for a backpack unless that is your life style
Use whatever baggage you like. Some people find a small backpack handy for carrying their necessities during day trips. These may include sunblock, mosquito repellent, drinking water and tissue paper to be used in lieu of toilet paper which is not always provided in public toilets.
Your hotel room may have only one free electric outlet for charging your electronics. Problem solved if you bring a small power strip from home. Note that in Thailand electricity is 230 volts although the plug is often the same as in Canada.
Millions of tourists over the years have used old fashioned suitcases, some still do! I use a wheelie now. For walking around the city, on tours and shopping a small pack or small duffel bag can be helpful.
You will need something smaller for a carry on bag anyway. For my carry on I use the smaller bag for my computer, books to read, spare clothes in case I get stuck somewhere. Also for meds, hand cleaner, maybe snacks along with the small plastic baggie that holds your allowed liquids.
Up to you.Edited: 06 August 2018, 22:38
Reading some posts would be helpful...Many posts have been written on this subject...
Thanks everyone! Your advice is all very helpful and appreciated.
Day pack is nice when out and about. But as far as main luggage, anything works, as you'll probably go from; plane/airport - taxi - hotel, in AC comfort. What it's packed in, is irrelevant, unless you actually plan on lugging stuff around on foot.
Then a backpack would be handy. Although, I've yet to carry luggage further than the car / taxi to hotel door. Why would you ?