I have learned the most about Excel from using it and pushing it to its limits. They may sound silly and simple, but it's true. Take any workbook you have and find a way to make it better. Someone else on A71 asked me how to learn more about Excel recently (if I could recommend specific books, courses, websites, etc.) and here is what I told her:
The best I can tell you use to bookmark Google and find a project you want to do, then figure out how to do it. I'm entirely self-taught with Google's help, so don't know of any review courses, but know you can learn a lot with Google.
Finding challenging projects before it's crunch time at work can be a challenge though, so just start with what you have. Do you have a spreadsheet where you track your bank account, maybe with budgetary categories? If so, try to find ways to extract and use data. Think you can set up a formula to calculate expenses in a budget category in the current month (without having to reset the SUM range)? How about setting the budget column to turn red if it exceeds the budget by 5% or more? Maybe make another sheet that pulls data from the bank records and uses pivot tables and charts to show expenses by category, payee, etc. Or maybe you want to create a formula so that you can type a payee in A2 and see YTD expenses… Next maybe you try creating a macro so that when you export data from your bank's website, the data is format to copy and paste into your bank spreadsheet. But the bank doesn't have your budget categories, so maybe you want to use vlookup or index and match to create some weird formula that looks for the last time this payee was paid and tells you what budget category it was.
These are just random ideas off the top of my head, but things like this are how I learned Excel. After all, necessity is the mother of invention! Most of all, though, no one will ever know everything about Excel, so the best thing to do is to learn how to learn with it (aka Google). My biggest things to say are super useful and you should be sure you're familiar with are filters, conditional formatting, vlookup, and macros. Pivot tables can simplify your life a lot too. These might be things you're already comfortable with, but if not, they're well worth playing with. None of these things are really scary just take a little bit of familiarity. VLOOKUP scared me to death the first couple times I used it but now I use it often for simple things just cause it's so handy and easy to use!
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