Hello ab620, welcome to the forum.
Since your wife is a teacher you guys might be interested in doing some work with kids at one of the NGO English schools there. You didn't say how long you would be there, many of them want you to stay at least a month to be effective with the kids. All the kids want to learn English and they love to have native speakers to help them. Some of of the schools will let you stay at a place provided by the school, you pay a little, not very fancy, but cheap. Don't know what you and your wife like to do, but interacting with the people there can often be more rewarding than just seeing the tourist sites with other tourists, as amazing as some of them are.
Since you said you are going to see both Siem Reap and Phnom Penh you might want to take the speedboat along the Tonle Sap River between the two cities, a good way to see the river life and how some live along and on the river there.
I am jealous of you going to Koh Rong, we have been to Cambodia many times over the past decade, sometimes staying several months at a time, but have yet to go to Koh Rong, it is certainly on our list, will probably do it next time.
Angkor Wat and some of the other sites are certainly worth seeing, but if you see just the tourist sites you will not really see Cambodia nor what the people are really like. Visitors who only have contact with people who work the tourists sites will hate Cambodia, visitors who have a chance to see the real Cambodian people will love Cambodia!
The summers are actually not too bad if the rains have started they cool things down a bit. And don't worry about exchanging money, dollars are accepted everywhere, in fact the dollar is the defacto currency everywhere in Cambodia, the Riel is given only for change (they don't use or accept U.S. coins) and for purchasing low cost items. Cambodians use the dollar. However be careful that the U.S. currency you bring, especially $100 bills are in excellent condition, not too worn out, no marks or tears or they won't accept them. The exchange rate is about 4000 to the dollar not 2000 as was mentioned in another post. They are not quite as particular about the quality of $20 bills as they are for $100s, I don't know why, but take only good looking currency. ATMs are available in all the cities, but not yet in the countryside, while we take some cash with us, we mostly use ATMs.
You and your wife will have a great time.