Is it important to pre-book flights as you travel around Thailand or can you buy tickets a day before you plan to fly? Thinking about Bangkok to Chiang Mai and Chiang Mai to Krabi

It depends on your budget.

I took a look here Bangkok to Chiang Mai | Google Flights

Click the Date Grid to see the lowest fares for the next few days. The fares go as low as $42. Keep in mind that that the low-cost carriers charge additional fees

It is about 10AM on Thursday in Bangkok and I can get a ticket for a BKK-CNX flight at 5:35 PM today on Bangkok Airways. The price shown is $128

Click on that flight and on the next page scoll down to see a link to book at the airline website. Click through and you can book online the fare is 3910 THB or about 109 USD. That same flight tomorrow is about $140

Voetjet would be $90 to $170 today depending on departure time and prepaid options.

If those fares are in your budget then you could book and fly today. If not, then two or three days in advance would cut the price significantly.

Click on the link where it says this:
The cheapest time to book is usually earlier, up to 3 weeks before takeoff
Prices are currently high

Since prices do tend to be cheaper booking a couple weeks ahead, and you have some flexibility in your schedule, I'd recommend checking fares over the next few days and snagging tickets if you see a good low price pop up. That way you've got your flights booked without having to wait till last minute and risk higher costs. Safe travels!

Pre-purchased airline ticket on Nok Air to Bangkok but cannot see where we can purchase check in luggage!