If you've uploaded an excel spreadsheet to google drive and forgot to tell it to be converted, or auto-saved it from your email, it's not obvious how to actually convert it afterwards. To do so, just right-click on the spreadsheet and under 'Open With' select 'Google Sheets'. That will auto-convert it and open it up, with the converted copy also added to your drive.
It's not the most technical protip, but it took me a while to find it. Hopefully this will help someone else too.