Google Slides have lots of templates built in. If you want something different, I recommend Slides Carnival.
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Help with all things Google.
Visit the Chrome Web Store for tons of free extensions, apps and themes. I regularly use bit.ly, Eye Dropper, One Tab, The Great Suspender, Black Menu for Google, Save to Keep... but there are tons more.
To create interactive and engaging lessons and level-up your Google Slides, try Pear Deck. This is a paid service but the free trial is great and can be extended.
For engaging and collaborative verbal feedback, try FlipGrid.