Welcome to Siem Reap, Cambodia
Siem Reap is magical. Although the initial draw to the city is to see the fabled Angkor Wat, Siem Reap is a destination to be considered on its own account. It is a bustling, gritty, infectious city that I fell in love with at first encounter. Big enough to be lively and small enough to be completely unlike Bangkok, Siem Reap is my number one "return to" destination in Asia. Give it at least a few days; although I imagine you will want to stay longer.
Obviously. Spend a morning here; I don't think it's necessary to buy more than a one day pass. The "thing to do" is to wake up at 4:00am and take a tuk-tuk to watch the sunrise over the temple. The tuk-tuk driver then becomes your chauffeur for the rest of the morning/afternoon. Generally it costs about $20 for the driver. Round up a group of four people and you get a personal driver waiting for you after each temple for $5 each.
Walk down Pub Street at night and make your own alcoholic fruit smoothie. Walk around with it and get a drink and play games (Connect Four, darts, pong) at Angkor Famous. Then, go to Angkor What Bar and dance your ass off. Go all in and f*** it, get a bucket.
Of course there are many many other bars in Siem Reap but I speak only of what I know.
Eat your food on the street - it's better and cheaper. Try fish amok, a typical Khmer dish. I would say to have Cambodian bbq but you can read about the unpleasantness of my experience with it here.