What is a significant event in the Russian Revolution that I could write a diary entry about from Tsar Nicholas' II point of view?
From the Tsar's point of view, I doubt he really believed their whole system of government could be overthrown, so a perspective he might exhibit is either ignorance of the situation or arrogance of power. You could focus on the events in St. Petersburg as the city fell to the Bolsheviks and the people and units of the navy joined with them.
I would take an almost philosophical approach, and focus on his personal reactions. What would he focus on? What would be be interested in following? Where would his emotional connection lie. That would be the most interesting aspect of this assigment for me. It requires you to take what you know and determine a personality.
I suppose another thing you could write about would be the overthrow of Kerensky's government and the rise to power of the Bolsheviks. You could write about whether you think the Tsar would have approved more of the provisional government -- if he would have thought that such a government would have been better or worse for the country than a communist government.
In my mind, this is a fascinating activity. Tsar Nicholas II is one of the most complex historical figures. There are so many elements about which you can write and explore in a fairly complex manner. Contrasting Tsar Nicholas' desire to lead Russia and his people with the aloof and oblivious manner of their plights and predicaments would be an excellent undercurrent in any diary exploration. For example, discussing his reaction to the Bloody Sunday protests and demonstrations could serve as an excellent basis for a diary entry. If Nicholas loved his people and valued his position as leader, why would he open fire on them in such a brutal manner? What must have his reactions been as he recognized that protesters were chanting "God save the Tsar," while they were being mauled by the dragoons and guards? Exploring this would be profound because it had to represent one of the moments where Nicholas felt immense complexity in emotion and perception. Given how his reign ended, he had to have had some thought or reflection that if he were more skilled at addressing the needs of the Russian people earlier on in his tenure, he might have won greater favor with them.