As if long periods of swinging lazily in a hammock, punctuated by the excitement of taking a four-metre plunge into the Mekong, weren't enough to occupy anyone, you can also rent an inner tube for 10,000 kip and slowly drift down the Mekong for a few hours.
People usually start at the northern boat landing or their sunrise-side guesthouse and go as far as Santiphab on the far side of the French bridge. Apparently the currents get stronger after that, so it's wise to wrap it up there. You'll have to make arrangements for someone to come down a pick up the tube if you don't want to haul it back about three kilometres overland.
Kayak tours are available for 180,000 kip, which includes a spot of fishing, a visit to some waterfalls and lunch.
Every evening before sunset, a sunset cruise leaves from the northern landing to take in the view from the water. The cost is 60,000 kip and you'll need to ask at one of the shops near the boat landing where to go precisely.
You can also easily get to Don Khon by crossing the French bridge, where there are great waterfalls, rusty trains, and the occasional glimpse of an Irrawaddy dolphin. See Don Khon activities for more info. The best way to get here is by bicycle as the distances involved make walking impossible for all but the most avid of walkers.
It's also possible to book other tours: elephant riding in Ban Khiet Ngong, mountain bike treks, and so on. And if there's something in particular you want to do, someone will be willing to hook you up for the right price.