You will have to fly in/out from bkk to get to Luang Prabang or Vientianne. The other alternative is to take a flight to Chiang Mai, bus it to the borders at Chiang Khong and cross the river to get to Laos. From there you can take a slow two days boat down the river to Luang Prabang or you can bus it to somewhere else before heading down to Luang Prabang.
Things to see and do depends on how many days you have and what you are interested in. If you are the outdoors type, there are some interesting hikes in the North parts of the country. Great for photos too with lots of local villagers. It is more mountainous in these regions, so travel is slow and not easy. You can also do some kayaking in Vang Vieng. Luang Prabang is nice to wander around for few days. You can try out local massages and herbal steam baths.
In the central regions, you can check out the Bolavan regions for coffee/tea plantations and some hikes.
Down south, you can go ride around the many islands in Si Phan Don, going through paddy fields, saying "Sabaidee" to the friendly locals. And then sit back and relax in the hammock by your little bamboo hut and watch the sunset at the end of the day.
It is a third world country after all. Expect slow and some hard traveling. Plan for a longer visit, move at a slower pace and you will be able to enjoy the beauty of this lovely country.