While it’s a better idea for a long weekend, I highly recommend Nong Khiaw just north of Luang Prabang. It’s about 3.5 – 4 hours ride via minibus.
You can either book minibus passage in town or take the public bus from Luang Prabang’s northern bus station. The town has a laid-back vibe, with many options for activities, including visiting nearby caves, kayaking, and trekking.