Once in the "Activities" view, click on the "Star" icon, to the right of the title of the activity you want to add to Favorites

From your homepage, you can find your recently added activities in "Favorites"

If not, have a look to this Article : How to customize the "New Posts" block.
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To organize your Favorites, you will find all the important information in this Article:

How do I organize my Favorites?

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