I went there in 1996 when travelling was still discouraged so I’d imagine things have changed a bit.
Pnomh Penh is quite charming, despite the shell marked buildings, and you can easily spend two or three days there. Stomach turners are the mandatory trip to the killing fields and (forgot the name) the high school that was used as a political prison. Check out the FCC also for a taste of the colonial life.
The other place I visited was of course Angkor. I took a boat there from PP but flew back. Why? Because although the high speed boats afford fabulous views of the Tonle Sap they were extremely narrow and dangerously overfilled - fear of capsize for 4 hours! Still they may have changed this given the amount of attention Angkor now gets.
There’s little to do in Siem Reap (outside Angkor) but you should buy at least the 3-day pass to savour the beauty of the huge temple complex.
Things to buy: markets in PP are good, especially the Russian one. I bought silks but they have a good choice of silver too. Pot can be legally bought everywhere - it’s a Cambodian cooking ingredient - but the quality isn’t much. A good deed is to go to the shops selling handicrafts made by landmine victims. They have some beautiful things, especially the leather products.