I am going to Bangkok and Im not sure of my return date back to Chiang Mai yet,

so I was wondering if I can just show up at DonMuang airport on either (Sunday May 1, Monday May 2, or Tuesday May 3) and buy a reasonable price air ticket to Chiang Mai?.

I would buy a round trip ticket with a flexible return date. Buying at the airport is probably the worst option imo.

Just checked AirAsia and ThaiSmile from Don Mueang.

Although 30/1/2 is a long weekend it does not seem to be booked too heavy.

Also in this direction not much booking to be expected.

End of long weekend means much trafiice towards Bangkok.

So generally I am optimistic.

I would buy a round trip ticket with a flexible return date. Buying at the airport is probably the worst option imo.

In the case of Thai Smile it would mean the difference of 3080 Baht vs. 1050 Baht.

32 (!) flights from DMK to CNX daily.

One of THE superhighways in the air in Thailand.

A lot would have to happen if all booked.

I would buy a round trip ticket with a flexible return date. Buying at the airport is probably the worst option imo.

In the case of Thai Smile it would mean the difference of 3080 Baht vs. 1050 Baht.

32 (!) flights from DMK to CNX daily.

One of THE superhighways in the air in Thailand.

A lot would have to happen if all booked.

so what is the cost at the counter like the OP wants to do?

I only have one such experience with Thai Smile.

There you could get the "Smile Saver" tariff at the counter but not the "Smile Promo".

For the example above that would be 1180 Baht vs. 1050 ("Promo").

I don't know about the others like AirAsia.

In the internet: AirAsia offers flights on May 2nd from 790 to 1189 without checked baggage.

15 kg +300, 20 kg +330.

"Flex" option cost about 1200 extra.