Later today we'll have CF Round #698, and one of the problem-setters is triple__a. I could not help but notice that he already has set problems for one contest, so I decided to give it a try and participated in a virtual contest. It was a Div. 2 Round # 630, but I still think you can make some useful observations. You can watch mine in the YT video.
tl;dr
Be careful with constraints, some of them are unnecessarily tight, in my opinion. Topics are standard: three implementation problems for div. 2 only, some number theory and combinatorics, dp on a tree, and later more advanced stuff. There was also an unusual problem about finding the bug in the given code.
General observations
The modint class helps a lot, and you should have it. A good sleep schedule is crucial. Do not write contests straight after you wake up after a short sleep, or at least be very careful. Finally, you can save a lot of time if you just keep solving problems instead of complaining about them :upside_down_face:
Unfortunately, such preparation does not always pay off, but hey, every contest is an opportunity to learn your weaknesses and fix them afterwards. Today's screencast will be here once processed