Ooh ooh - let me add some stuff. I teach a beginning Office class at my community college, and there have been some good things already mentioned but I have some more. Here’s what I cover in my class:
Basic arithmetic, of course, but also know some of the more common functions: =MIN, SUM, AVERAGE, MAX, etc.
Know how to copy and fill cells. Look at Excel’s Flash Fill - it can be pretty smart (sometimes).
If you’re copying formulas between cells, you MUST learn about absolute cell references ($A$1 for example). My example in the class is for a table of employees and their pay rates, if one cell shows a pay raise at 7%, how would apply that to all of the employees? I think it’s interesting that the $A$1 style (with dollar signs) is a holdover from VisiCalc!
Inigo mentioned VLOOKUP, which is really great to know. Think of tax rate tables, for example. I use them all the time in my grading by setting 0 to 60 as an F, 60 to 70 as D, etc.
Learn how to do text concatenation, either with a =CONCATENATE function or by using the & operator (my preferred method).
Know how to import a file of text data, split text into multiple columns, and sort by column
Learn the basics of charts: pie charts, line charts, bar/column charts, and scatter X-Y charts, and where they’re best used. Know how to format and modify the chart title, axes titles, backgrounds, etc.
We also work through an example that I use from a summer camp signup roster in Excel. It has columns for names, addresses, cities, states, and zip codes, along with their T-shirt sizes. From that we generate address labels (and more) using Mailings from Microsoft Word (shows some of the integration capabilities). Then just for fun, I show them a simple pivot table that counts the number of T-shirts for each size (with two clicks).
I don’t do a lot with formatting (it’s pretty obvious), but I cover conditional formatting as well as some of the printing options.