Don't take the "speed-boat" in Laos from Huay Xai to Luang Prabang or if you do, do not sit at the back of the boat next to the engine, it has the least amount of leg room and of course an engine next to your ear for 6 hours or so.
Do rent a motorbike and travel around the hills if you go to Huay Xai... Can't remember which direction it was but I believe if you turn left after you walk up the hill from the border post in Laos across from the Thai border.
Do go up to the waterfalls in Luang Prabang if you are heading there.
The bus from Luang Prabang to Vientiane can be a bit hairy too, driver had to drive around a little land slide and almost went off the cliff