I can only speak to what I personally use in cover letters, so here goes. I aim for short and sweet- none of these paragraphs is more than a few sentences long.
First, I do an introduction that includes who I am, what position I am interested in, and if applicable, where or who I heard about the position from.
After that I do a short paragraph about my experience- school, outside jobs, other experience that I think applies. Sometimes I include a current project, and particularly if I'm applying for an out-of-town position, I include the area where I'm located.
Then a paragraph of any questions I have about the position- this is where I would ask about what they want included in a portfolio. For an internship, I probably wouldn't include too many questions- those would be great to ask if you get an interview. And if the job requires something that I think I am uniquely qualified for, I include that as well. (I like to respond to information that was in the ad for the position- such as "I read music, and have experience with large cast operas." if that's what they were looking for.)
Finally, I do a very brief conclusion that tells them that my resume and references are attached, and that they can contact me for any additional questions, etc. And I thank them for their consideration.
I don't put too much info in my cover letters- I save most of it for an interview. I've never had to make a portfolio before, but if it was me, I would simply ask them what they were looking for, and work from there. My big things for cover letters are short, sweet, and polite. Hope this helps, and good luck!
