Enter...
Enter and Explore Angkor
Bicycle
Bicycles are a great option, although it will take more time to get around the temples, the journey is peaceful and you will have time to take in more of the atmosphere of the complex. Rental is cheap, around $1 per day, make sure you look around for a good quality bike.
Motorcycle
Motorbikes can only be self-ridden around the Angkor complex if they have been rented outside of Siem Reap, a great choice which gives total freedom around the park. If you are looking to rent a bike locally, you will have to rent one with a driver, as this is the law in Siem Reap. However, the cost is low around $6-$8 per day, and you driver should give you some interesting information about the temples. Make sure they are licensed, and have a grey numbered vest.
Explore...
To enter the Angkor site you will need a pass. These are available at the front gate and can be purchased to best suit your plans. Make sure you only buy your passes from the official Apsara Authority counters, not from anyone else, new or second hand.
Passes can be purchases in a variety of options: 1 day for $20, 3 day for $40 or 7 day for $60. The 3 day pass can be used for any 3 days within a week, whilst the 7 day pass is valid for any 7 days within a month of purchase.
If purchasing a pass in the evening, you can enter the park after 5pm, to view the sunset without using a day of your pass. Passes are non-transferable, have your photo on them and will be checked regularly around the park.
You can hire a guide for around $20 a day and are available speaking most major languages. It may be a good idea for you to hire a guide for the first day at the complex to help you get orientated.
Make sure you get to the temples early in the day. The park opens at 5am, whilst the temples open soon after at sunrise. Naturally, there are fewer visitors earlier in the day, so stay in front of the crowds and tour buses.
Taxi
It is also possible to hire a car with driver by the day. All drivers will be familiar with the area, and may even offer you suggestions they make speak little English – However an easy, air-conditioned way to get around. A licensed English speaking guide (and driver) will cost you in the region of $45-$50 per day, a great option if there are a few of you.
Tuk-Tuk
You can also arrange a Tuk-tuk from your guesthouse, a novel way for two to travel to the park. The cost should be in the region of $12 for the day if you are visiting the main temples. Again, like motorbike taxis, the driver should be licensed and wearing a grey numbered vest. You will pay a little more to get to the park for sunrise or visit the outlying temples.
Helicopter
Helicopter tours are a stunning (but expensive) way to view the Angkor temples. Flying over the ancient ruins is more than just impressive and will give you more of an idea of the scale of the Angkor complex. Prices start at around $90 for a 14 minute tour. Helicopters can be chartered or booked in advance and flights leave daily from Siem Reap airport.
Elephant
Elephant rides and horse carriages are available within the park. However, this seems to be more of a photo opportunity for the riders rather than any serious temple exploring, as there are only limited routes.
Tour Bus
If you want comfort take a guided tour bus around the temples. Beware: You will be exploring the complex with a vast crowd of people and may only get to see the main attractions and miss out of some of the interesting other ones. Expect to pay $25 - $70 per day.