Frankfurt is a great place to get started on a trip, because both local and distance trains have stops right at the airport. Like all good German trains, they are on-time, frequent, clean, and well engineered so they run smoothly.

There is a lot of discussion here in other threads about Mainz, which is a smallish city about 1/2 hour away. Depending on your direction, it could work for your first stop. Among other things, it has some interesting Roman ruins. Just do a search on "mainz". For that matter, try the same technique on any other place than strikes you as a possibility. Tons of opinions and facts here.

Romelle