I'm visiting friends who have recently moved to Germany, about an hour east of Mannheim. I'm looking to see what there is to do/ see/ experience while I am there. I'll be going in mid-September for 1 week. Transportation is no concern as I will be taken around by my friends. This will be the first trip of, I hope, several while they are stationed there, so must-sees are great, as well an off-the-beaten-path would be wonderful! Thank you for any advice, it is appreciated.
Heidelberg and Mainz are worth day trips. I also like Stuttgart, but it sounds like you are a week or so early for their Cannstater Festival.
Please tell us what you think you might like to do: castles, museums, hiking, etc.
Depends on what town you start from. But Rothenburg ob der Tauber might one option for a daytrip and its maybe a must see
Some other nice places might be Schwäbisch Hall 1,5 hr east of Mannheim on the Autobahn A6
as well as the Tauber Valley near Rothenburg with Bad Mergentheim and villages in the vincinty like Weikersheim and Markelsheim little more than 1,5 hrs northeast of Mannheim Try the local wine "Tauber Schwarz"
Heidelberg is nice, also, Strasbourg, France, the Black Forest (Triberg and Friberg). Trier and Vianden in Luxembourg as well as Lux city.