Within Google Apps Docs and Sheets, Add-ons are available. You can add whatever you think will be most useful to you. While I haven't experimented with Sheets much yet, Google Docs has several really useful Add-ons. (If you ever worked with Scripts in the Google Apps of two years ago, Add-ons pretty much replaces them, though if you have an existing script in place, it will sometimes still work.)
I took a look at TextHelp Study Skills that allows you to highlight text in Docs. (Click to make pictures bigger.)
You may also insert everything from Venn diagrams to organization charts to floorplans with Gliffy Diagrams.