The classic intro adventure U1: The Sinister Secret of Saltmarsh is a seminal work in the history of D&D. Placed on the fringe of the kingdom, this cozy town is an excellent place to launch a small regional campaign. The module is a haunted house mystery that then spins off into further adventures. The town of Saltmarsh itself has a notable character, and it got further development in more recent editions of D&D.