Udon is a sizeable city not small. Plenty of traffic. Rather ugly place. Plenty of bargirls there but low quality. Really can't see the appeal. Nongkhai is ok but limited. The other places get boring quick. If single all these towns in Isaan are fine for short trips but long term rather average. Korat has some appeal but no airport which is strange for a large city. Khon Kaen is ok with airport links to north and south.
I think Chiang Mai, Rai, Phitsanulok or a beach town is the go. Apart from the smoke the north is very good. The food is better, airports (huge plus), pretty girl. Lots of road trips.
Beach areas - anywhere from Hua Hin to Nakhon Si is good. Jomtien to Chantaburi is good.