Multiple sentences inside parenthesis

Solution 1:

I would punctuate around parentheses the same way as dashes.

With dashes, the sentence would look like this:

This morning I was distracted by a plane — or was it Superman? I'm not very sure — so I tripped.

Therefore, with parentheses it would be:

This morning I was distracted by a plane (or was it Superman? I'm not very sure) so I tripped.

That said, I prefer dashes here, because the middle phrase clearly relates to the topic of the sentence. Parentheses are better reserved for signaling a stronger digression from the main topic, e.g. —

This morning I was distracted by a plane (they're built in a factory the next town over) so I tripped.