The blocks around Pub Street (French Quarter) in Siem Reap contain an abundance of restaurants of every which type, from Khmer to French, Italian, Indian, and American.  Many of these restaurants offer French style second floor balconies overlooking town, and open air cafes on the street level.

I encourage to explore the alleyways of this neighborhood and find the place that best suits your tastes.  Just east Pub Street you will find street vendors selling food (rice, noodle dishes) for 1 USD.

Here are a few mentionables:
Khmer Kitchen: Probably the best Khmer food in town.  Make sure you order up the Amok, a classic dish of fish stewed in coconut milk.
Red Piano: Check out the pictures with Angelina Jolie on the wall, and order up a Tomb Raider cocktail at this trendy joint.  Try to get a seat up on the second floor balcony.
Angkor What?: One of the oldest pubs in town.
Temple Club: Popular bar with outdoor seating area.  Beware of the pitchers, as they are watered down, so make sure you order beers from the can.
Swenson's: If you're in the mood for a good American Ice Cream parlor (and in the sweltering heat you certainly will be), check this place out in the shopping center along the river just northeast of the Old Market.