Faraer's cited the most important sources on the Stonelands. IMHO, they are a great setting for adventures. Some ideas to keep in mind:
1) The Stonelands are certainly home to many secrets. Ancient strongholds such as Irongard ("Iyarim's Guard"), the Caverns of the Claws (home to troll warbands), old bandit keeps aplenty, and perhaps even some dwarven and goblin fortresses abound. In addition, the PCs can get involved in intrigues involving the Lords Who Sleep (see the 1e FR boxed set; essentially, the LwS are a group of powerful knights kept in stasis against the possibility of a diabolic invasion of Cormyr), or encounter rogue nobles, assassins' hideouts, or strange artifacts or ruins there.
2) The Stonelands are a turbulent front for a many-faceted struggle waged between the Zhentarim, the Purple Dragons, and the Harpers. The Zhentarim traditionally run caravan routes through the Stonelands, seed the region with safeholds, and carry on a campaign to disrupt competing trade through Cormyr, as well as generally destabilize the Forest Realm, by organizing monster attacks and stirring up conflict along the edges of the region.
As such, there are many, many ways your PCs can find adventure in the Stonelands. For one thing, the Zhentarim already have their own bases in the region, so anyone coming in and setting up a new stronghold is likely to be seen as a threat. For another, the area is rife with "dungeons," in the form of ancient ruins, deepspawn lairs seeded by the Black Network to create monsters to disrupt trade and harry Cormyrean patrols, and Zhentarim caravan bases; the PCs could spend their entire careers exploring and clearing out these places. Another possibility is that the PCs become deputies of the Crown itself, with the mission of doing what has hitherto been impossible for the Purple Dragons and creating a Cormyrean stronghold within the Stonelands.
