Kompong Speu province stretches from Kandal (suburban Phnom Penh) to the wilds of the Cardamom Mountains. Clockwise from the north it is bordered by Kompong Chhnang, Kandal, Takeo, Kampot, Koh Kong, and Pursat. There is not much to see and what is to see is usually seen as a day trip from Phnom Penh.
 Kirirom National Park
Disclaimer: I have not been here. Kirirom is about 100 kilometers from Phnom Penh and is one place in Cambodia that affords safe hiking opportunities. There is an old hill station here and you can spend the night at the top. The Rough Guide Cambodia gives the most thorough write-up of the park for what you can do and how to get there.
Udong
This on-again, off-again capital from the 17th to the 19th centuries lies about 40 kilometers north of Phnom Penh on an excellent highway. It's tucked away in a little corner of Kompong Speu practically in Kandal province. It is almost always visited as a day-trip from Phnom Penh (really a half-day trip). Don't concern yourself with accommodation here.
As it's easily reached it is worth a few hours of your time. There are a number of stupas dotting the hills ranging from about three years old to several hundred years old. Several kings are said to be buried here. Walking along the ridge there are a number of shrines and other structures including one that was used as a munitions depot by the Khmer Rouge and was resoundly blown up. Both the Rough Guide and Lonely Planet Cambodia guides give more than adequate write-ups of what the different stupas, shrines, and structures are. | 
