The National Parks are worth visiting, but the problem is rustling up the numbers to make it affordable.
A better option perhaps is Tha Khaek from where you can visit the spectacular Konglor Cave and other stuff -- and there's no need to rustle a group up.
savannakhet is probably my least favorite of the lao cities i've visited. having said that, it is far from completely charmless. you can walk along the river, enjoy a good evening at one of the lively restaurants, etc. it is a long way from vientiane to pakse by bus. since you've got the time, i would definitely stop to break up that trip, and either savannakhet or thakhek are the obvious choices. have fun. cheers.
I like Savannakhet a lot and it is easy to spend a couple of nights there. There is much more to do than in Tha Kek which ios a bit of a hole. The only reason to visit the latter is if you want to see the Konglor cave mentioned by Somtam. There is a great circuit you can do from Savannakhet, visiting he monkey forest and a temple storing ancient Pali manuscripts. Further you should see the dinosaur museum in the town itself. I wrote a thread on Savannakhet here some time ago. Use the search engine.
Sava is definitively a lovely town, it just takes some time to discover what makes his charm. The tourist information near the river has some excursion map for free, you can do the trip to the Pali bibliotheque by motocycle, and it's a new beautifull japanese coffee shop : cafe anakot, on the main road (Ratsavongseuk) wich at his own is worth a visit of the town, with very good japanese-lao fusion food in family style. Just take it slowly and enjoy ;o)