Of course, everyone goes to Siem Reap for one thing, the majestic temples. I suggest that you visit Angkor Wat at sunrise. Yes, you will probably have to wake up at 4 am, but waiting in the dark for about 15 minutes as the sun comes up and starts showing the temple is one of the most gorgeous sights you will ever see. Once you finish enjoying that, you can walk over and go into Angkor Wat and explore. If it is your first time, it is worth it to book a guide who can walk you through the place plus give you extensive information and the history of the place and all other temples you decide to visit. You can usually do Angkor Wat super early, stay a couple hours then head straight to Angkor Thom for the Bayon Temple, which you will probably recognize by the many smiling faces you will see in it's exterior. Allow another couple of hours to explore, admire and take photographs.
A bit of a hassle is that they are restoring a lot of the main temples, so you will probably see scaffolding and construction if you go this year (or even until 2016) but no matter, it's still an awesome experience knowing that alot of them were built in the 9th-13th centuries! Can you imagine all the hard work that took especially since there was no modern machinery yet! Ta Phrom, or the Tomb Raider temple is going through major renovations right now and I am a bit disappointed that they built platforms in the popular photo op areas because it looks so touristy now compared to last time. Nevertheless, it's a must see temple and considered one of the main ones to see, especially if it's your first time.
It's was my first time to fly to Phnom Penh this last trip, which is the nation's capital aka "the big city," according to those we talked to in Siem Reap. To be completely honest, my first impression of PP is that it was so hectic, condensed and dirty (like piles of garbage everywhere) It seemed to be a city that was still emerging, with their first real mall opening only a few weeks before I arrived. It takes awhile to warm up to the city, especially if you get used to the laid back vibe of Siem Reap (and also people seem to be friendlier since there are more tourists in SR) but once you get passed the initial stress of the traffic and confusion of street addresses that don't make sense and tuk tuk drivers who always say they know where you want to go but really don't, you will find the charm in this city, mostly through little hidden alleyways, restaurants and museums. Do not miss the Royal Palace (pray it does not rain, or if reports say it will, save it for another day because we got stuck inside for over an hour because of waist deep flooding outside) and the National Museum for historical relics and the like. Also, it may not be the happiest place on earth but the Tuol Sleng Genocide Museum is a must see, especially to learn more about Cambodia's more violent past (Don't all countries seem to have one? Except like, Switzerland) I had to hold back tears as I looked at pictures of the 20,000 people held prisoner, tortured and killed in that school turned prison and interrogation center.
I would give around 4 days if you are visiting only Siem Reap and a week if you want to do both SR and PP. Here's what to expect in terms of prices when you are there:
Transportation from airport-hotel: $7 for a car/ $10 for a van
Tuk Tuk- SR: $1 day/ $2 night and PP: $2-5 depending on distance.
Bus from SR-PP and vice versa: $13 for the night bus (8 hours on a bed for 2 so make sure you are with a friend OR book both unless you are ok to sleep next to a stranger!
Meals: $5-10 per person (with drinks)
Tuk Tuk Rental: $8 half day/15 whole day
Hotel: $40-50/ night for a decent hotel through Bookings.com or Agoda.com
Drinks: $2-4 Cocktails and Wine