As far as Cambodia goes, I would say a couple days hiking in the Angkor Wat complex is a must. The nature is beautiful as is the sense of exploring history. Siem Reap is a nice town with lots to explore in terms of history and night life. (PNH has its own charms but it's a much more complex place and not all that great for in-and-out tourism)
The main tourist hotspots in the South are Samui, Puket, Pipi, Khao Lak (Pang nga province),Ao Nang(Krabi Province), and Lipe(satun province). All the major international chains are represented in all or many of these locations.
As far as I recall, Puket may be the best bet for planning island hopping excursions due to higher departure volumes.
I hope this helps!