Three week Cambodia/Vietnam - Nature/history focus

I am trying to plan a three week trip to Cambodia and Vietnam, with a focus on history and nature. Most of the online itineraries seem to be more city/touristy focused, I'd like to see some of the big cities and tourist attractions, but would prefer more rural areas and be able to go on some nice hikes and such. Are there any must see places to visit? This would be in January.