the airport in PNH is quite near the city, so the drive should take 30mins max. with the traffic. I'm not sure about the Grand Palace opening times, if it's open, you could have a stroll there (it's a beautiful complex, like the grand palace in bkk, just smaller) and then have lunch at fcc balcony or some other place on the riverside. the palace is a block from the riverside cafes.

the visa is $20, bring passport photos, no tea money problems at the airport (crossing overland into cambodia is a different story). the airport is small, but nice for cambodian standards, i think there's a lounge but don't know about the details. if you go out of the airport you'll have to pay the departure tax of $25.

have a good trip and enjoy angkor!