There is a slew of add-ons that can increase the functionality you already get for free in Docs and Sheets. You can locate them or activate them by just clicking the Add-ons menu while editing a document.
Among the best: Avery Label Merge, for importing addresses from Sheets into Docs for mail-merge print jobs; EasyBib for creating bibliographies and citations;
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While Gliffy is for inserting diagrams and flowcharts; and Table of Contents for quick creation of a clickable ToC to use for navigation.