To get an idea of how Cambodia was like during the French colony it's the best to get to Kep. I would call it the posh Cambodia. You will be surprised of the cleanliness and the European influence in architecture and infrastructure. You'll even see trash bins and fancy cars everywhere. It's just 30 km away from Kampot and easy to get there. If you just follow Rd 33 towards Vietnam you'll get there in just 40 min. Take a ride through the paved road by the beach and enjoy the cool breezy weather of the south coast. There's not that much to do in Kep that you have to stay there overnight. Just do a day trip from Kampot and you'll be fine. Arriving from Kampot you'll notice various restaurants by the beach selling fresh sea fruits from the Gulf of Thailand. Have a seat by the water and watch the fishermen do their job. The main dish of Kep which you really have to try is crab. There's nothing more delicious than fresh crab from Kep with Kampot pepper. Mmmh... yummi!
If you plan to stay in Kampot overnight then you should better get back before it will become dark. The road is good though but you don't want to get stuck in the dark if something happens to your motorbike. Trust me!