Phmom Penh - is the capital of Cambadio with the histoy of the killing fields. Central market. 

Battambang - Bamboo train

Krong kampot national park = very touristy

Sihanoukville has a beautifull beach

Siemreap - where the Angkor temples are
Phom Kulen national park

Mondulkiri - hills and active activities and elephants! 

Koh rong sanloem (book ferry 24 hours in advance to and from Koh rong and bring money!)- island in front of the coast of Sihanoukville, it is very pretty. 
secret beach named clear water bay.
THere's a jungle route which you can find near Eco sea beach which will take you to clear water bay (30 minutes). 

M'pay bay (on Koh rong island)
Swimming with the plankton :O

Angkor Wat ofcourse