How to answer that without sounding extremely negative.

So I’ll start with saying I enjoy living here, appreciate everything Thailand has offered me, an inexpensive retirement, for the things that matter to me.

Get 2nd thoughts every now and then, the reality of living back in the states snap me back to reality. As an east coast yank, vacationing here, it wouldn’t even be a consideration, as too far and expensive to get here, and yanks usually don’t get enough time off to take advantage of the inexpensive daily cost.
Now, if you have too much time or money, and seen the rest that interest you and want to see a little countryside, then it can be a relaxing experience. Little research is required, as I found out, there isn’t a whole lot here that really interest me, or anything that I would consider ‘world class’, though sure the TAT would disagree with me. I’m not a big fan of public transportation, from experience, bus/train travel doesn’t interest me, and car rental, would probably be equal or exceed you total daily expenses for everything else, hence why many of the non tourist areas are just that.
The north side, I enjoy kanchanaburi, I enjoyed the tak/mae sot, then north route to mae hong song. There is a huge dam/reservoir just north or tak, for water activities, seen it, but never experience it, it is huge. It will take quite a bit of time, little planning and need to not mind being in a car for few hours, as can turn into a long day between towns offering accommodations. Sept/oct would be the recommended time to travel for me, better vis, no burn off of old crops as in feb/mar/apr, decent weather, everything still lush and plenty of water in rivers/waterfalls. Could loop around via Pai, sort of a KSR in the countryside, not my kind of thing, but many enjoy it.
If I was to repeat the trip, bkk/nakhan sawan/tak area, then up along the border, (not route 1). Up the border to mae hong son, that will require a stop half way. Then the northern side of MHS, the road gets even slower, hillier and windier, so easy going, a stop at Pai, not my thing, but many like it. CM or CR did nothing for me, haven’t seen Lamphun or Lampang, though seem interesting. I would stop at sukhothai on the way back, if history interest you, also kamphang phet was an interesting town, there is one really nice guesthouse overlooking lake/river, rather nice, can be seen from the east side of river. Can be seen on the way or on return as between bkk/tak. In that aspect, you can go up route 1, and return via border. The rest of the area really didn’t or doesn’t interest me. As stated, went to golden triangle, just because it was there and nice museum, but very logistical just for that. Would save that for another time. Golden triangle, then back along the river, khiang khong, head south, thru nan / leoi, then decide which way to return, via river again to chiang khan to nong khai, or straight away over to udon thani from loei, then down via korat stopping at phimai and phanon rung, 2 more restored ruins before heading back to bkk.
Those are some of the cities I’ve been to that interested me, roi - et was nice also, but a bit off the beaten track for a special trip. But again, all are going to take some time to get to, and depending what you are looking for, may or may not enjoy. I’m an R&R person, so easy to make happy. Give me a beach, lake, river to sit next too, and I’m happy, throw in a pool, good inexpensive food and drink, and I’m in heaven. A good night market is a plus anywhere.
North side has rafting and trekking opportunities if it interest you, days past for me, plus already done in much more pristine place. One advantage of living in states/Caribbean/mexico area, some primo locations. If not working for an airlines, and on a fluke, last minute vacation, I would of never discovered Thailand.
Just some thought, hope it wasn’t too negative or off putting. Been around the country twice, almost, and would really not recommend too many things as a special trip. I enjoy my beach time, with couple side relaxing trips during the year. But that’s about it. Hua hin to phuket is fun time for me, the rest is filler and R & R. But 9 months in udon thani a year is plenty. Happy to leave, happy to return. If already here and have plenty of time, definitely get out and see the countryside, have no regrets, obviously, as done it twice, 1st time a bit faster, 2nd time, taking more time and a closer look at things, with a lot more info. Don’t book anything too far in advance, as if you could in some of these town, as some may interest you for short week, or one night might make you want to move on.
Happy travels.