Sorry for the delay as just came back and have to go reservist. Anyway, starting a business in the rural or semi rural areas are also another possibility. The town you choose need to be big enough (for the population) and potential to grow (if you really want your business to take off)
Some towns you could consider are Pattaya, Phuket, Koh Samui, Kachanaburi and Maesai.
I am currently staying in Maesai, thus I would focus on this town which I knew best.
Maesai is the most northern town in whole of Thailand. About 74km away from Chiang Rai (city) in Chiang Rai state. The town borders Myanamr's Thachilek. Over here, most of the core businesses here sells jeweleries and stones (raw). Maesai used to be an important gem stone trading post (till now it is) but its already lost much of its stone traders to Kachanaburi and Bangkok. This is mainly because Myanmar's Mogok rubies are actually running out. Moreover, just in year 2000, Maesai was bombed by shells fired by Myanmar's Junta chasing the Shan rebels. So after the scarce, most of them actually closed shop.
Other shops are mostly catered to tourists. There used to be a lot of farangs or foreigners would come here monthly to do their VISA run (30 day stamps) but since the october 2006 ruling of limiting 90 days limit. But the local Thais from the south still frequent here in the cool season to have their holidays and shopping. So the town is always crowded from Oct till May.
Maesai is developing at an exponential pace as it serves as a trade portal for goods from Myanmar and China. Already, they are building another bridge (just near my house) for the trucks of goods and imports.
Opportunities here are great. If you have a lot of capital you might want to consider doing an import/export business. Provided you are sure what are the things that Thai's would want to buy.
Law and order is average. No better than those in the city but the good thing is that most people here are definitely more friendly and helpful than those in the cities.
If you plan to retire here and start a small business to earn some extra cash. The easiest would be to open a small shop selling clothes or a restaurant. Rental is consider cheap for shop houses furthur away from the main road. While rentals at the main road could be exponentially expensive. (10,000 baht for a small unit)