Three or Four Week Laos Itinerary

Laos offers nature and history galore, all on a very small budget. If you have more or less than three weeks, adjust as you see fit and make sure you budget extra time for transport!

Cross the border into Laos from Thailand and spend a night in the border town of Huay Xai.
If you have more than two weeks, bypass the slow boat to Luang Prabang and take the bus to Luang Nam Tha where you can trek in pristine forests and hang out with ethnic tribes.
From Luang Nam Tha, rent a motorbike to drive up to Muang Sing to explore the Chinese border and trek through tribal villages without many tourists.
Organize your transport for Nong Khiaw where you can relax and enjoy peace and quiet in your bungalow along the Nam Ou river.
Head upriver to check out Muang Ngoi, though keep in mind that it now has electricity.
[Take the boat from Nong Khiaw to Luang Prabang] for a more scenic and shorter route than the slow boat everyone talks about.

Soak up the scenery in Luang Prabang while volunteering to teach English to monks and local students.
Take a bus from Luang Prabang to Phonsavan to explore the mysterious Plain of Jars and learn about the Secret War.
If you have time, head to out-of-the-way Vieng Xai to visit the caves the Pathet Lao leadership lived in during the Secret War. (This trip is also possible from Nong Khiaw, but it means you have to skip the boat ride to Luang Prabang.)
Make your way south towards the 4000 Islands, and add a pit stop to take a boat 8km through a cave in Kong Lo.
Settle down in either Don Khon or Don Det, and check out Irrawady Dolphins, waterfalls, and the view from your riverside bungalow.