The central region cuts a swath through north-central Thailand -- stretching from Kanchanaburi bordering Burma in the west to Sa Kaeo bordering Cambodia in the east. Known as the ricebowl of the country, when the rice is in the ground, expect to see almost endless emerald green paddy sliding by your bus or train window.
With the exception of the province of Kanchanaburi in the west, Central Thailand gets relatively few backpackers and tourists, with most heading further north to Sukhothai and the northern cities or east to Cambodia. To be fair, there is a good reason for this as aside from lots of rice cultivation and the occasional well regarded temple, there really isn't that much happening from a tourism perspective in this region.
Kanchanaburi is the exception to the rule with a wealth of natural attractions, including national parks, spectacular waterfalls, trekking opportunities, relaxing riverside scenery and loads of good places to stay. It's also known for a somewhat obscure chapter of World War II that saw over 100,000 Allied prisoners of war and Asian forced labourers perish while constructing the "Death Railway" under the forced supervision of the Japanese army.
For those with more time, strike further west along the spectacular Route 323 to the fascinating town of Sangkhlaburi in far western Kanchanaburi province. A mixed Burmese/Mon/Thai outpost, Sangkhlaburi boasts an entrancing lake and some terrific trekking potential. Enroute you'll pass the blip of a town of Thong Pha Phum -- another spot for the more intrepid traveller looking to get off the beaten trail.
The more north-central province of Lopburi runs a distant second to Kanchanaburi in the popularity stakes. A historic town with a bunch of ruins dating as far back as the 12th century, Lopburi is better known for its monkeys. Overfed and with a generally bad attitude, these simians virtually rule the town, and with an annual festival held in their honour, it seems that they're in Lopburi to stay.
As for the rest of Central Thailand, sit back and enjoy the scenery -- if you do decide to jump off your train or bus to explore further, you'll be in unadulterated Thailand without a tourist in sight. Both in the northwest province of Uthai Thani and the northeast province of Nakhon Nayok there is a lot of potential for nature lovers, but so far it's mainly popular with Thai tourists.