What should I do when visiting Cambodia?

Cambodia is a fascinating country, with some of the friendliest people in the world. Siem Reap and Phnom Penh are the most frequently visited, but the country receives enough tourists that there are tourist facilities nearly everywhere.
• Siem Reap. The tourism capital of the country, located near the huge temple complex of Angkor Wat. Actually, Angkor Wat is only one of hundreds of temples in this area. I spent 3 full days touring just the Angkor Wat park (which comprises most of the most important temples near Angkor Wat as well as Angkor Wat itself) plus an additional day touring temples further away as well as the floating village on the Tonle Sap Lake, and still wanted more time here. Definitely worth spending at least 4-5 days here, though some people find that temple fatigue sets in after a couple days.
• Phnom Penh. The capital of Cambodia. Though dirtier and grittier than Siem Reap, it does have its charm. It is generally safe to walk around, just be aware of your surroundings and watch your stuff. Visit the Central and Russian markets, but be prepared to haggle! Walk along the river in the morning and watch the locals doing calisthenics. The Royal Palace is worth a visit. On a grimmer note, the S21 (Tuol Sleng) museum and the Killing Fields are depressing reminders of Cambodia´s recent past. S21 is rather harrowing and not for the squeamish, but the Killing Fields are eerily peaceful.
• Battambang. Cambodia´s second-largest city, it is known for its French-era architecture, statues, temples, and the bamboo train. The train is really just a small bamboo platform with a small engine, but definitely worth a ride if you are in Battambang.
• Kampot. A small town near Sihanoukville on the coast. It still has a quiet small-town air, though increasing numbers of tourists pass through it every year. Plenty of accommodations and expat-run restaurants and bars. The real draw is the surrounding countryside. Hire a scooter and ride out to Kep, exploring the caves and the Secret Lake along the way.
• Sihanoukville. Beach town full of backpackers partying and drinking on the beach. Some people love it, some people hate it. I have to admit I am in the latter camp.
• Kratie. A small town on the Mekong, it is a nice place to stop on the way to one of Cambodia´s eastern provinces. It is one of the best places to see the Irrawaddy river dolphin.
• Ratanakiri and Mondulkiri Provinces. Still the least touristed parts of Cambodia, it is not worth visiting unless you are willing to spend 3-4 days exploring the countryside and possibly visiting the surrounding villages. Multi-day trekking or village visit packages are offered at all the guesthouses in these areas. If you are willing to rough it a little, these are highly recommended. Don´t bother if you are only there for one day though- you will get only a superficial view of the place, and it won´t be worth the travel time to get there. It is possible to explore the region on your own with a motorbike, but this is not recommended unless you are reasonably proficient riding one, as the roads are not always in good condition and go up and down a lot of hills.