It makes sense to plan for the first few weeks in PhD as it will help ramping up for the new life as a PhD. Following are the few tips recommended by people who went through this period:
Vaguely identify in few lines the problem you are trying to solve and the research area that you plan to work/publish in.
List the courses that you think are relevant to your research area. Also, list top 10-20 research papers according to how relevant they are to your research area first and how many citations they have second. Discuss with your professor on what good papers and courses are missing.
Keep a log of what you do and find daily. Make notes at multiple levels of granularity (e.g., one sentence summary of the entire paper all the way down to sentence-level notes) from papers you read.
Literature Review justifies the reason for your research and allows you to establish your theoretical framework. It will also help organize the learnings from the papers.
Get an idea of who did what in your group. Learn how a PhD thesis is structured in your group.