Well, Pattaya inspires the same kind of love-it-or-hate-it division that Las Vegas does, for slightly different reasons. Both places are unlike any other in the world in their respective categories, and with size comes other advantages like cheap rooms in low season and lots of expat-chef restaurants with cuisines from all over the world. Walking Street has become a real "thing" for tourism (not unlike the Fremont Experience, come to think of it).

June might be as good a month as any to take a shot at Angkor. Absent a war or something like that, crowds will only go up, and at least they'll be well off the peak size. Any other place in Cambodia could be considered.

Ayutthaya can be done as a daytrip from Bangkok and has a good historical park.

Hua Hin is a less-hectic beach alternative to Pattaya that can be reached with a train ride.

I'd consider Luang Prabang in Laos as an alternative to the golden triangle area, which has gotten tamer and more touristed in the last two decades or so.

Ko Samui is way, way overbuilt, IMO, much like Phuket. I saw it in 1995 and 1998, and also Ko Pha Ngan back then (and Ang Thong Marine Park). You can have a nice trip, but the chase for the less-developed spots has moved on to places needing combo airline-and-ferry tickets such as in Trang province (but: Andaman side) and a few down the eastern seaboard. I dropped out of the chase after seeing western Samoa and the Seychelles.

The thing you have today that wasn't around 25 years ago is all the cheap LCC flights everywhere. Bangkok (BKK and DMK) has a good selection both domestic and regional, though KUL is still the champ as the hub. That can be both a blessing and a curse, though, as those flights have caused places like Luang Prabang and especially Angkor to get many more visitors than before.