Most visitors land in Bangkok and spend a few days getting acclimated and touring the many sites of the Thai capital. Bangkok also makes a good hub for trips around Thailand and the region. Itineraries will be limited only by the amount of time you have, and many find that travel in Thailand and the region of Southeast Asia is best done on the slow. Most short-term visitors head right for the beaches, but below are a few options for touring. Hit Chiang Mai or Bangkok markets on the last days of any trip to stock-up on souvenirs for the folks back home.
Ancient Capitals Tour--Starting in Bangkok, trace the legacy of the ancient capitals. First, north to Ayuthaya, capital until the late 18th century, then via tiny Lop Buri (best by train) to Phitsanulok then Sukhothai and Sri Satchanalai, the very origins of the Kingdom of Thailand. Many carry on to the north and visit the vestiges of the Lanna Kingdom, a culture that developed parallel with ancient Thailand and centered around now-booming Chiang Mai.
The tour is a short-course in the evolution of Thai architecture and many get hooked and make a trip to the archaeological sites in Isan, the Northeast, like Phimai or even to the ancient Khmer ruins of Angkok Wat in Cambodia.
"Adventure First/Beach Second" Tour--Many visitors like to "earn" their toes-in-the-sand lounging time. Here's a good way: Starting from Bangkok, hit the far north using Chiang Mai as a hub for rugged trips into the back of beyond; rent a motorbike and get muddy, take a trek, go rafting and then, tired and bedraggled, hop a flight to one of the beach destinations in the south, Phuket or Ko Samui for example, and put your feet up, spin some tall tales, and catch-up with that journal writing or get into that novel you've been toting around. This can be anything from a 2-week to a 4-month proposition.
Routes through other parts of the region are as varied as folks who travel them. Some start in Singapore or Indonesia and work their way up through Thailand and Indochina (even beyond to China via Laos or Vietnam). Another popular route is a circuit including Laos, Vietnam, and Cambodia.