Kaoh Ker was our absolute highlight. It is 2 - 2.5 hours away from Siem Reap, so most people skip it. We hired motorbikes for USD$8 per day and drove ourselves up there, stopping at Beng Melea on the way. We wanted to get a day of writing in at Beng Melea so we camped overnight, but you could do this as a day trip if you don't mind a lot of driving.
Beng Mealea is a temple on the drive to Kaoh Ker. Most tour groups will stop here to break up the drive to Kaoh Ker. We loved this temple so much that we spent an entire day there doing some writing, and decided to continue onto Kaoh Ker the following day.
This temple is in ruins and reclaimed by the jungle. You could whizz through the temple in 20 minutes if you're templed-out, or you could spend several hours there if you like to take your time. Either way, if you love seeing nature reclaim and take over, this is a must-visit temple.
Easily! Either with a private driver or self-driven.
If you have a private driver, they will take you to each of the temples. You can organise a private driver either through your accommodation, or barter a price with a taxi driver on the streets. A private driver may suggest you visit "the best temples", but in our opinion - they're all the best.
We opted to self-drive. We hired motorbikes in Siem Reap for USD$8 per day and drove ourselves there. It takes a bit over an hour to drive from Siem Reap to Beng Mealea, and then a bit over an hour to get to Kaoh Ker. The road is good and paved the whole way. It took around 2.5 hours to get back to Siem Reap. We don't recommend driving in the dark in Cambodia, although if you do plan to drive late, make sure to bring your own night googles / clear glasses to see so that you can protect your eyes while driving, as the visors of hire helmets are usually so scratched that you can't see through them. You don't want to get a stone flicking into your eyes - or a bug flying into them! During the daytime, just bring sunglasses.
It took us two hours to visit all of the smaller temples at Kaoh Ker site, an hour for lunch, and then another hour at the massive Kaoh Ker Pyramid Temple. Four hours total. You can actually walk up the temple for an incredible panoramic view, although it's definitely worth walking around the base of the temple too and enjoying the grounds.
We noticed that most of the tour groups just went to the big Pyramid and skipped the smaller temples - which were the most exciting spots! At almost all of the small temples we visited, we were the only visitors.
Given that it takes 4-5 hours return to drive to Kaoh Ker, you may as well enjoy them all!
You can see the layout of all the temples at Kaoh Ker in the screenshot. You will drive in from the South - note the purple star on the left of the road. This is Prasat Pram, and consists of three temples next to each other and two more in front of them that have all been overgrown with tree roots. You must stop here! Then, when you get to the fork in the road, take a right. This road will take you to all the other temples. You do not need to use a map for this section - the temples are all located on this road and you can see them from the road. They are all clearly marked and impossible to miss.
By going counter clockwise, you will finish up at Kaoh Ker, and then just have two minor sites to visit on your way out. Enjoy!
Here is our exact breakdown of the money we spent over our two days at Beng Mealea and Kaoh Ker. Note that there's no accommodation here - we chose to free camp with our tent. Do be careful if camping as there are lots of areas with unexploded mines.
The total cost came to USD$133.38 for two people for two days, including bike hire, entries, and food. That's $33.30 per person per day.