On the south west coast of Cambodia, Kampot is hardly off the beaten track. However compared to the numbers of visitors to Siem Reap, Phnom Penh, and even the nightmare of development that's Sihanoukville, Kampot is relatively unvisited. This is surprising as it's a rewarding base for independent travellers, both as a town, as a centre of cultural activity, and as a base for day trips.
Kampot has good accommodation and eating options. You can gorge on Kampot pepper to your hearts content. It's worth noting that genuine Kampot pepper deserves it's reputation, with a unique blend of heat and floral notes.
The most common day trip is to Bokor Hill Station. Built by the French, then patronised by the Cambodian ruling class, it has significantly cooler weather, great views, and interesting high altitude tropical vegetation. Once famous for it's eerie deserted buildings this atmosphere is harder to find now, thanks to an outbreak of typical Cambodian overdevelopment. Think giant casino, helipad, and a somewhat tacky renovation of the Bokor Palace hotel.
On the other side of Kampot there are several caves and ancient temple complexes well worth a look.
Battambang is a regional capital in the central west of Cambodia, on the other side of Lake Tonle Sap to Siem Reap. An interesting blend of Thai and Khmer historical influences, it is reinventing itself as a cultural hub. There is a fascinating self-guided architectural walk available, which is a great use of a couple of hours. In addition to interesting possibilities within the town, Battambang serves as a good base for attractions in the surrounding area.
Bicycle tours of the area are good, particularly to Ek Nohm. to the south, Phnom Sampeou manages to combine some interesting and quirky temples with reminders of Khmer Rouge atrocities, plus nature.
Khmer sites outside Angkor Wat
Historical Khmer sites are scattered across Cambodia, not just at Siem Reap and Angkor. Some are stunning in themselves and well worth the effort to get to. Plus there is the bonus of far, far fewer crowds.
About 120km north east of Siem Reap this is a major site famous for it's stepped pyramid. This is surrounded by a plethora of smaller temples. The Hindu god of death, Yama, is big here.
Prasat Preah Vihar
This temple site is west of Siem Reap, sitting atop a cliff on the Thai border. An extensive site, this is remarkable for the location as much as any particular building remaining.
A car is the only really viable way to get to either of these sites. It is possible to combine the two in a single loop, with an early start and a long day.
If you're interested in Khmer ruins generally, outside the 'greatest hits' of Angkor, there are several very comprehensive web sites devoted to these.