For sure SE Asia has tons of things to see and do. Lots of old temples type of stuff.
Others who've been there that time of year can chime in but I think CM burning season falls around that time so you might want to avoid it.
I would probably say go to Siem Reap and enjoy Angkor. You don't say how long you plan to spend. But Phnom Penh can be interesting for a couple of days too. Depending on how adventurous you are there are less-visited parts of Cambodia too.
You could also just spend you time in Thailand. There is much more to the country than Bangkok (and CM). There are old Khmer temple sites in Isaan (NE Thailand). There's lots of national parks. There's Sukhothai.
I liked Luang Prabang when I was there a few years ago.
Myanmar is fascinating. But finding accommodations and getting around isn't as easy there.