Chiang Mai to Krabi
Direct flights connect Chiang Mai with Krabi, the gateway to Railay Beach and the Andaman coast's limestone-cliff scenery, without needing to route through Bangkok. Where direct flights aren't scheduled on your travel dates, a Bangkok connection remains a reliable fallback, usually with a reasonable layover on either of the well-connected Bangkok airports.
Chiang Mai to Koh Samui
Koh Samui has its own airport, and while direct Chiang Mai–Samui service exists on some schedules, it's less consistently available than the Krabi route — connecting through Bangkok is often the more reliable option, particularly outside peak season. Factor in that Samui's airport, privately operated, tends to carry a premium compared to other Thai domestic airports.
Planning a north-to-south itinerary
If you're building a trip that spans Northern Thailand and the southern islands, book this leg once your overall dates are set, and compare direct versus connecting options for your specific travel dates rather than assuming either is always better — availability shifts a lot by season.