My advice to you, oh lucky person, would be to:
1. Visit the line of crab shacks that hugs the coast for a sampling of local seafood. People swear by Kimly Restaurant. Go there. Order anything with the pepper sauce - crab (their specialty), grilled fish (below), prawns. You will not regret it.
2. Take a walk around the town. Kep was one of the last strongholds of the Khmer Rouge regime. My colleague, who is my age, weaves tales of growing up in the town, hiding amongst gun battles, and surviving on a meal of rice and potatoes. These days, the town is undergoing a wave of tourism, though it remains largely rustic. It will be interesting to see what happens to Kep in the upcoming years. Still, evidence of its war-ravaged history remains: in between swaying palm trees, on verdant hills, you'll glimpse French colonial mansions, decrepit and often bombed and riddled with bullet holes.
3. Watch the sunset from the beach, glass of kir in hand. Even on cloudy days, the sunset boasts wonderful gradations of purple, blue, and pink. Sunsets in Kep are one of my favorite things about Cambodia.