Kanchanaburi in west Thailand is the River Kwai day-trip with riverside seafood, Karen and Mon ethnic-Thai cuisine, and the Death Railway WWII history along the river.
Tip: Two-day trip is better than one. The Death Railway train ride from Kanchanaburi to Nam Tok is the canonical scenic stretch over the Tham Krasae trestle bridge.
Hua Hin on the Gulf of Thailand coast is the King's beach-resort town with night-food markets (Cicada, Plearn Wan), fresh seafood and the heritage station of the 1911 railway.
Tip: Stay overnight for the seafood markets. Cicada Market runs Fri-Sun nights 16:00-23:00; the Hua Hin night market every evening from 18:00.
Mae Klong Railway Market in Samut Songkhram is the famous market where vendors set up under the train tracks and pack up eight times a day when the train passes through.
Tip: Train passes through eight times daily; the busiest viewing slots are 08:30 and 11:10 arrivals from Ban Laem. Pair with Amphawa floating market.
Ayutthaya in central Thailand is the 1351-1767 former Siamese capital with heritage Thai kitchens, river-prawn stalls and the Roti Sai Mai cotton-candy-and-crepe market.
Tip: Take the morning train from Hua Lamphong; lunch at Pae Krung Kao river-prawn restaurant. Roti Sai Mai stalls are at the market opposite the train station.
Amphawa Floating Market in Samut Songkhram is the canonical evening floating-market day-trip from Bangkok, with boat vendors grilling seafood and Thai sweets Fri to Sun.
Tip: Weekend evenings only (Fri to Sun, 15:00-22:00). The firefly boat tour after dark is the headline; book at the pier for 60 baht per person.
Damnoen Saduak in Ratchaburi is the morning floating market 90 minutes west of Bangkok, sampan boats pile with tropical fruit, boat noodles and coconut palm sugar candies.
Tip: Morning trip only (06:00-12:00); arrive by 08:00 before the tour buses. Boat hire from the pier is 150 baht per hour per person.