Unter anderem / Unter anderen Dingen
In German, do you have to say "among other things" and not "among others" when the "things" are not defined (i.e. when the "things" haven't been mentioned before) ?
For instance, which one would you use: Ich habe von vielen Themen geredet, [unter anderem / unter anderen Dingen] Sport. [Unter anderem / unter anderen Dingen] habe ich entschieden, dass (...)
In both of your examples you would use "unter anderem".
Actually I'd say, that "unter anderen Dingen" doesn't really exist. Something similar that's quite common ist e.g. "Sie ist eine unter vielen Köchinen" (she's one of many cooks) oder "Er ist einer unter vielen Köchen" (he is one of many cooks). The only sentence I can think of, that contains "unter anderen Dingen" is: "Das Auto ist eines unter vielen anderen Dingen die er besitzt" (the car is one of many other things he owns", but the meaning differs from what you wanted to express.