The best known destination in the Khammuan area is Tham Lot Kong Lo - the Hin Bun river passes through this cave for seven kilometres, and the cave is as much as 100 meters wide. The surrounding terrain is karsts that put the Vang Vieng area to shame - but I didn't get there. It's off the main tourist track. Not that I mind going off the tourist track, but it often costs considerably more and I didn't want to pay the price this time. So I stopped in Tha Khaek on the edge of the Khammuan province (where they have no internet ... the withdrawal symptoms weren't too bad). I took a tuk-tuk with another couple Canadians about 20 kilometres into the area, and saw some fantastic scenery (above) and some wonderful caves (below).
The caves we saw were mostly large enough to have daylight streaming in, and had water in them - less of the scrambling through dark narrow passages I remember from Vang Vieng. They were also mostly unpopular with tourists, and thus very peaceful: except for the last one, we had every one to ourselves.
The Tha Khaek Travel Lodge deserves some kind of special mention: it's away from the road, and the common area is a wonderful inside/outside arrangement with seating all over, plants, and a small bonfire outside at night (it's quite cool there). A real blessing among the relatively cheap places I've been staying.