If you are going to Luang Prabang from Vientiane, definitely stop for a few days in between at Vang Vieng. Huge cliffs raise straight out of the ground and are surrounded by rice paddies and they are wonderfully scenic. There are a lot of fine caves there, too. You can also hire an innertube and float down the river for a few hours. Rent bikes and explore on your own. There are some fantastic swimming holes and caves nearby.
If you are leaving Luang Prabang, don't do the two day boat journey if going back to northern Thailand. Instead, go north in the back of a truck for a day or two and explore. If you thought there weren't many foreigners before, their scarcity now will come as a nice surprise. Then you can get another truck going back down to where the two day boats normally stop for the night and sleep there.
Resist the urge to get back to where things are "happening." I didn't spend as much time in Laos as I should have, I thought I was tired of the slow pace. But as soon as I got back to Thailand I missed the simplicity of traveling in Laos.