Cochem is the best along the Mosel for frequent day tripping... Rich loves Beilstein - but the connections are less freq in the evening so you may have a 2 hour tour with ice cream in Cochem while you wait like my buddies did one evening after a long day trip! In Cochem - I loved the "Lorespeicher" (sp?) that rick mentioned was a bit of a splurge - it was 80 vs 45 euros - cheap compared to big city spots at well over 100E and very quaint and friendly.
re: Bernkastel - I visited one day from Cochem but breifly and wished I had more time - some of the most famous vinyards (Doktor-Vinyard) and old town were amazing... go early and explore/hike the castle and vinyards and town!
I did Burg Eltz and Marksburg both and they are great as well. If you stay at Baccarac - then Markburg is closer. Burg Eltz closer to Cochem - but both just a 1/2 hour to 1 hour train ride away!