If I am writing .NET code I would often expose IEnumerable where-ever it could possibly make sense. Maybe it was down to LINQ and the fact you could us
Whether or not you want to be a Stream yourself, primarily depends on whether you're truly a Stream with the corresponding powers, i.e. support parallelization (not all Streams do, but it's one of the many benefits of Streams, and it's an important one). Normally I'd rather be a Collection and let myself be Streamed by the corresponding API calls.
The getStuff() method, returning Stream from myCollection.stream() looks right to me.