The main international airports in Thailand are in Bangkok and Phuket. These are well-connected to all the countries. All the airlines that travel to Asia fly to Thailand. So there is no dearth of flight options. Other international airports are located in Krabi, Koh Samui, Chiang Mai and Hat Yai. These are connected via flights from the South-east Asian countries. The major national carrier in Thailand is Thai Airways, followed by Bangkok Airways.
There are regular buses from Singapore to Hat Yai.
Thailand has a single international train service operating between Butterworth and Kuala Lumpur in Malaysia that extends to Singapore. The train journey is not preferable as it takes almost 48 hours and requires you to change trains twice.
There are six international border crossings from Cambodia. The busiest border crossing to get into Thailand is the Friendship Bridge across the Mekong from Laos. It is entirely possible to drive up to Thailand from Malaysia and Singapore, though not through rented vehicles. The main crossings between Thailand and Malaysia are Padang Besar and Sadao in Songkhla province.
Ferries ply between Phuket and Indonesia in the months between November and May. Cruises from Hong Kong, Singapore, Australia and Europe make regular stops at Phuket and Laem Chabang (Bangkok).