River Cruising
Trips on the Mekong and Tonle Sap rivers offer fresh countryside scenery and riverine life of the  locals. You can have boat cruises on the Tonle Sap, between Siem Reap and Phnom Penh, around Kratie. Near Kratie, having a boat tour to get a sight of the freshwater dolphins could be  a highlight of your trip in Cambodia. On the Tonle Sap you can go birdwatching in the nature reserve.

Trekking
You can go trekking around Ratanakiri and Mondulkiri which offer stunning mountainous scenery and colorful ethnic culture. Still there are are lots of unexploded landmines in some remote areas of Cambodia. Having a good tour guide is definitely necessary for your safety. While trekking, don't stray from the well-trodden paths.

Watersports
There's  a wide range of watersports including swimming and snokelling available at Sihanoukville.

Also travelling in Cambodia you can embark on motorcycling or biking adventures which travel through the less-travelled routes of the country.