Phnom Penh is a great city on the river and well worth a visit for a few days ..
Siem Reap and the Temples of Angkor are one of the biggest tourist destinations in SE Asia and attract millions of visitors every year..
And if you want something quite different head to the coast in Cambodia and check out Sihanoukville and Kampot and some lovely quiet beach areas along the coast..
You can even travel overland by bus from Sihanoukville back into Thailand via Ko Kong along the coast..
Interesting trip and onwards to Trat Thailand and even Koh Chang island Thailand and back to Pattaya or onwards to Bangkok.
I have been over most of Cambodia and it's a fascinating country and well worth a very good look.
The world is your oyster in Cambodia, Vietnam and Thailand and years of adventures to experience if you want to 😁