If you're staying in Thabok (or Vientiane - it's not that far really) you're very close to the NBCA Phou Khao Khouay where there are some of the nicest tropical monsoon forests in Bolikhamxay province and perhaps in the whole country. It's only few kilometers off the RD 13 before the town of Thabok.
One of the most attractive things to see beside forest, butterflies, birds, trees and plants are Phou Khao Khouay waterfalls - and I really enjoyed Tad Xay fall (there are 2 more but less exciting). It is very peaceful and lush place filled with insect and bird sounds from the forest, there are many butterflies on the rocky shores and it seems like a perfect place for picnic in front of the fall that falls over the black rocky steps about 30 m deep into dark and deep basin. I saw few people swam there, some others made BBQ in shade but it was far from being crowded. It was a nice and hot day out to seek refreshment in the shade. The riverine landscape further on from fall is wonderful, dark rock continue along the river and in dry season you're able to walk by its water which sometimes disappear under the rocks and then reappear within few meters ahead. Superb sonic and visual experience! Few hundreds meters ahead the water will fall once again into very deep gorge another 40 m deep and there probably your way will stop.
To get to the Tad Xay area you'll have to follow signs and turn into jungle road - dry season is no problem but in rainy it will become very difficult one (4 WD!). A small fee is paid to the guard upon entry to that area. Some agencies in VTE can arrange your visit with the guide.
Before you go to Thabok and contemplating pending a few days there: no nightlife in town and so far no western foods in roadside restaurants. If that's important for you, book one day trip to falls or NBCA through some of the VTE based agencies and return in VTE the same day. Thabok is quite nice, but there's nothing on after dark.