"Entertaining" is an adjective and you can use "so" as a modifier to express how entertaining whatever you were doing or watching is or was.
This probably explains it better than I can:
I agree, there's definitely two conflicting usages here: and activity can be the noun "fun", in the same way that doing something can be can be "bliss" or "hell" or "a complete waste of everyone's time". And so while I can say "My drive in to work was hell", I can't say "My drive in to work was so hell" (because not an adjective). But if I can say "My drive in to work was enjoyable", and even "My drive in to work was so enjoyable", then what's wrong with saying "My drive in to work was so fun"?