I've done the drive with and without a GPS. It's definitely a better trip with a GPS (and I can read the road signs). I bought my GPS (Garmin) in Chiang Mai for under 4000THB.

I do not see this as an interesting or memorable journey. The benefit of a car is the ability to come and go as you wish, and being able to find the out of town places (visiting friends, attractions, etc).

If you simply are looking for an interesting drive, my advice would be to rent a car in each, both Bangkok and Chiang Mai for a few days. The north is beautiful, and has some great drives. There are also many trips within a 4 or 5 hour drive from Bangkok.
Also understand that road travel in Thailand isn't without risk. The highways are mired with accidents and hazards. Drivers are not known for their courtesy or terrific driving skills. And that's not even including the high drunk driving rates. FYI.