ok so looking at Singapore/Cambodia over Xmas New Year
Normally have a holiday in SE Asia about now but unable to this year.
Cambodia and Myanmar are the only countries in SE Asia I haven't been to previously so not a novice but far from an expert either.
At this stage looking at 3/3/3 Siem Reap/Pnohm Penh/Beach somewhere.
Personally I would forego the beach but I'm outvoted and feel I should play along as 3 days of temples is going to stretch any goodwill I have banked.
Bear in mind my backpacking days were before most of you were born. Having said that we don't go 5 star luxury either and more than happy to immerse ourselves in the food and culture
cambodia is cool. few years since i was there so it's probably all changed since then, but cheap as chips ($1 dorm and 35c beer at the time), nice folk and some amazing shit to see.
the temples in and around siem reap are sweet, but take some time as there are loads. temple fatigue will kick in and stretch that goodwill as you assume. as per all SE asia, there are bike/scooter/tuk-tuk options - some of the further options are out of reach without motors. angkor etc get busy, and hot during the middle of the day as pretty exposed.
the boat trip from siem reap to battambang is worth considering, leisurely, pretty, floating villages etc - and lake tonle sap provides pretty much all the protein in the country so it's interesting from that point of view too - but takes extra time. battambang itself is only really a one day place i think - has a training circus which is a bit of fun if it's on when you're there, some bats and some batshit - the bamboo train is a good albeit brief laugh. dunno whether it's usual or not, but we saw a load of monks gathering alms a la luang prabang (but without tourists) there too.
obviously everyone's experiences are different, but phnom penh was just a big asian city to me - the khmer rouge stuff excepted - so personally i'd knock that off quick and move on. killing fields is a must, and the audio guide there is excellent, pretty nasty stuff.
beaches - sihanoukville is a backpacker party town. messy, loud, drugs n booze. nothing wrong with that of course, but if you're after something a bit quieter then maybe catch a boat over to koh rong. it's not as spectacular as some of the thai / malay etc islands, but it's one of the two best phosphorescent plankton places i've seen, which is pretty sweet for a night swim.