Siem Reap and its surrounding Angkor Wat temples are certainly no secret anymore but you can find your own little secrets there.
You should spend at least some days clambering around the ruins of Angkor Wat. There are so many temples that it might be difficult to decide where to go. Climbing the Bayon temple where you can see so many serene stone faces or scrambling around the twisting roots of Angkor Thom in the jungle should be on your list. Looking and searching in some lesser known sites you might stumble as well across archaeological diggers.
One thing to avoid the crowds is to go very early or stay around until sunrise to loose these crowds. Visiting Angkor in the monsoon season had been a good idea to find yourself alone, but due to better hotels and rising numbers in visitors this is not an option anymore. There are always a bunch of people around. The most visitor are coming from Korea, China, Japan and they come mostly in big groups - hopefully you can dodge them one or the other way as they intend to be very loud, unmindful and you’ll have to wait a long time to be clear of them again.