Incoming intern, which Excel and PPT basics to know?

Excel: sumif(s), countif(s), v-x-lookup, index & match, RRI (for CAGRs).  Think about functions that help condense large sets of data.  For example, turning a trial balance into a P&L, or a P&L into a summary P&L.  It's not complicated formulas that make it happen, just proper organization and tagging that will allow the formulas to work.  Additionally, consider that your work will be heavily audited and overly complicated formulas aren't always conducive to that (i.e. index/match).  When summarizing across tabs it's good to get into the habit of adding excessive "Check" lines that when working should 0 out and will otherwise call out errors.

PPT:  If you can get good at Slide Master, you can gain a ton of efficiency.  Particularly with CIMs, if you invest time upfront creating a clean template it makes the iterations so much easier and allows for formatting consistency.  Try creating a title slide, various content slides (i.e. 1/2/3/4 exhibit versions), section dividers, etc.  Each slide should have consistent boxes for title, sub-title, bullets (including sub-bullets), footnotes, etc.   In terms of formatting/content ideas, STEAL STEAL STEAL.  Leverage past work, investor presentations from public companies, and any/everything else you can find.

To be clear, I don't think this stuff is critical to know as an intern, but since you asked...