Haha, the SIPA essays were actually not too bad once you got down to it!
I applied for the MPA in Development Practice. If it helps (since its due today!) - for the last one, I decided to focus on skills I had gained through my experiences (academic and extra curricular) that would be helpful for my goals. I guess this was sort of given, because my undergraduate major (computer science) is very remote from what I want to do now, but still gave me a unique skillset that I wanted to put forth somewhere in the application.
I found their admissions blog quite useful btw, have a look if you have time. I'm not very comfortable talking about a poor grade/gap in the application (I feel it just draws attention to it), but the website mentions "Part three can focus on things you are proud of, or perhaps not so proud of".
Good luck!