Hiking in Doi Inthanon by oneself
 

So while in Thailand, we're already taking 4 guided tours and would like to do some hiking by ourselves. Is this easy to do in Doi Inthanon? We would take a bus or mini-bus to the park itself from Chiang Mai...or we could rent a car. How easy is the latter to do in Thailand for a US citizen? And if we rented a car, would we be able to find the park easy enough by ourselves?

If you can rent a car it's probably easy for you , because it's pretty far to walk from one place to the other place.
otherwise you need to hire minibus to get there about 1,200 - 1,300 THB
For hiking should have guide.

Car rental is easy in Thailand for foreigner but you should have the proper international driver license or permit. Even with insurance, if you don't have the proper license your policy will most likely not be valid.

Driving to national parks in Thailand is a good idea. Note that most national parks have several entrance and exit points so I would do the research beforehand on which to use. GPS are available and even the maps on smartphones are a great help in finding directions.

We rented a car and spent a night in Doi Inthanon on our trip last year. We arrived in the park from Mai Sariang which was a nice drive through great scenery but the return to Chiang Mai was less so, but pretty straightforward along major roads for the most part. As Hanuman says there are several entrance points so a good map or GPS would be really useful. I used and old LP pocket atlas of Thailand, Lao and Vietnam that I have had for years, a bit small but ok.

Driving in Thailand is pretty straight forward but driving out of the centre do CM is a little daunting if it is your first time so you would be better collecting the car from the airport and returning it their and maybe getting a bus or taxi into the city when you are done.

Most of the waterfalls and the summit are definitely best reached by car and the park HQ has some decent info and maps. Can't help with the hiking as we save that for other places in the north. A little more detail on my trip report

http://www.fodors.com/community/asia...n-thailand.cfm