Driving here isn’t as bad as it is made out. Even Bangkok is relatively easy to drive through, just a busy city with bad signage, and bizarre one way systems.
Sukothai and Ayuthia are nice, if you are into ruins and all that, take a look at Phimhai, a nice little sleepy town.
If you want to go off the beaten path, Loei province is really nice, up by the Maekhong, go to Chiang Khan town, head up the mountains.
Head west into Tak province and there is a huge waterfall there that is a days elephant trek away in the wet season
You shouldn’t need an IDP if you have a uk photo card license, haven’t needed them for a while now, although I know lots of tourists like to have them.
Get a car and go explore makes life so much easier on holiday here.