Take post # 4 advice - do not miss the Buddha Park !!! Nice ride there on the # 14 bus for 4000 or so Kip, this place is a real trip.
Wat Si Saket is an absolute must, even if you are out-templed from Thailand.


Go out on old HWY 1 about 25KM across the Tha Ngon bridge to the floating restaurant. Order food, get on bamboo boat, float on the Nam Nguem and eat. Jump in the water. On the way back, stop off at the market and see all the bush meat/strange animals for sale - don't buy any.
See if there is a football match at the national stadium.
Go to the Lao textile museum. Dye your own silk scarf.
Rent bikes, head out towards the bridge, veer right at the clock tower and just go.
Get a tuk tuk to the Mekong Breeze restaurant -- past the airport, straight along the river as LP road turns right. Good Lao food, mostly Lao clientele, bad karaoke later at night, including great war-era patriotic songs.
Walk out to Don Chan island - follow the path straight off the footbridge until you come to the small cafe - buy beer and whatnot, find a plastic table along the river.
Eat lunch at the LiaoNing Dumpling Shop - Chao Anou Road and Samsenthai.
Find Kung's Cafe and have breakfast.
Go to the Sokpaluang Swimming Pool and take a dip (just past Wat Sokpaluang, on every map).
Get a tour of the Beer Lao factory.
Spend time at Wat Sisaket in the afternoon light.
Go to the military museum. Go to the Kaysone Phomvihan museum.
Find a place to play badminton and get your ass kicked by Lao people.
//Find a place to play badminton and get your ass kicked by Lao people.
Very true. However, I feel like I was Reggie Bush playing American football with the local high school kids.

I quite enjoyed Vientiane, even though there is not much to do. Renting a bike and just bike around was what I did the most. Buddha park is ok too. I quite enjoyed the national museum and all it's communist propaganda ("the american imperialists and it's cliques") and the exhibition centre across the road was having an exhibition on North Korea when I was there that was entertaining.