I have to write a diary as either Antigone or Creon after each scene totaling 4 diaries being a page long each.
The easiest journal entry to write would be the scene between Antigone and Ismene. Assuming you've ever had an argument with a sibling or a close friend over something you feel strongly about, you know how it feels to be so frustrated that you simply want--at least for that moment--to disown them. Antigone clearly wants to leave Ismene in the dust because she sees her sister as weak and unwilling to stand up for what's right. Since Ismene was willing to deny their brother, Antigone is willing to disinherit her sister. This would make for an explosive journal entry.
I think you might have a good activity if you wrote from both perspectives. Antigone's journal could address her emotions and the questions in her heart and mind with regard to why Creon won't allow her to bury her brother. Creon's journal could be about his emotins and questions about why Antigone would question his authority so blatantly. His journal would parallel and almost be an answer to hers.
Whether you choose to write from Antigone's perspective or Creon's for this scene, the most important thing to focus on for this journal is not re-telling the plot details as they happened. Rather, the focus of a journal is the feelings and emotions of the characters. Scene 1 should therefore be one of the easier scenes, as the events which take place both evoke very strong emotions in both Antigone and Creon.
Interestingly, both characters at this point in the play are very angry. The audience immediately sees Creon's anger--however calm and collected he wishes to present himself--when he makes his decree about leaving the corpse of Polynices alone. At the same time as this decree is made, Antigone is out burying her brother. She too, is likely feeling rage and sadness over the entire situation.
To write a page on these emotions, I encourage you to put yourself in the shoes of either character and really think about how you might react under similar circumstances.