Sure, it's great that you're venturing to explore Thailand's offbeat path. Here are some travel tips for Nakhon Pathom, Udon Thani, and Surin:

Best Time to Visit:
Generally, Thailand's cool and dry season between November and February is the best time to visit these places. However, if you want to attend the Elephant Roundup Festival, make sure to visit Surin in November.

Route and Accessibility:
All three cities are well connected with Bangkok via road and rail. Udon Thani also has an airport with regular domestic flights from Bangkok.

Altitude and Acclimatization:
All the cities are roughly at sea level, so altitude sickness or acclimatization should not be an issue.

Accommodation and Booking:
There are plenty of accommodation options ranging from budget to luxury in all three cities. During major festivals, book your stay in advance.

Clothing and Essentials:
Since all the cities are situated in the tropical region, lightweight cotton and linen clothing are perfect. Make sure to carry a good sunblock and insect repellent.

Health and Safety:
Food and water hygiene is extremely important in Southeast Asia. Stick to bottled water, and ensure you're up-to-date with routine vaccinations before the trip.

Local Etiquette and Respect:
As in all of Thailand, it's important to respect local customs and religious traditions. This includes dressing modestly (particularly in temples), avoiding physical contact with someone's head or pointing at things with feet, and always asking permission before taking someone's photograph.