The best road to take from Danang to Prao is the QL14G, the 14B will bring you 50km south of Prao to the Ho CHi Minh road and is quite busy with local traffic and some unpleasant road works. When you arrive after 80km in Prao you can take the Ho Chi Minh trail south to Kontum through the Annamite Range (Dãy Trường Sơn) or North to A'Loui (100km) and Khe Sanh (around 200km). The road between Prao and Khe Sanh is just one of the best things to do when you are looking for adventure around Danang. With 200 km of winding mountain roads and pure fun!
Especially with some heavy rain on the way you will have the fun to see all young ethnic kids playing on the road like a big party.
To be honest Khe Sanh is not much fun, there are no bars not many places to eat something worth mentioning and only offers you some cheap hotels to stay overnight.(around 250.000 vnd/night) If you are interested in the Vietnam-American war you can visit an old American combat base 13km outside of Khe Sanh, where some heavy fighting took place during the summer of 1967. For more information please see:
Wikipedia: Khe Sanh.
For me the best thing is the HCM road to Khe Sanh, it is probably the better part between Prao - Aloui - Khe Sanh.