The Rheinland-Pfalz is west of Hessen, and easily accessible by train from Frankfurt. It's the home of German wine, along the Rhine and Moselle valleys. Similarly to Hessen, the Pfalz is a hilly region with river valleys. The Pfalz is the ancestral home of a lot of German-American communities, including the Pennsylvania Dutch.
My first trip to Germany was in 1992, with my high school German club. It was a three-week exchange with a school there, in the town of Schifferstadt, which is a sister city with my home town. There's an old house built by a settler from Schifferstadt.
Speyer was founded by the Romans and is home to the Cathedral, a UNESCO heritage site. Various diets during the Reformation were held here. A much more famous diet was held in Worms.
Mainz is home to the Gutenberg Museum, which has a variety of antique printing presses and old books. Mainz was also founded by the Romans. The Rhine-Main region was strategically important for them.
Easily my favorite place in the Pfalz is Trier, the oldest city in Germany. It was founded by the Romans, and there are still ruins you can visit. The Black Gate, the baths, and an amphitheater are available for your viewing *without* having to go to Italy. Trier is home to one of my favorite juxtapositions: the palace and basilica. Trier is extremely out of your way, unless you're going to Luxemburg, but it's worth going if you can squeeze it in.
The Palatinate Forest, south of Kaiserslautern and southwest of Worms, is a nature park, and there's hiking and that sort of thing. We went to Bad Dürkheim while I was there.
Near the Rhein-Pfalz is Saarland, which is largely industrial and not very touristy to my knowledge.
Up next: Baden-Württemberg.