Fragments are one option. Another is just to use regular Views. There are libraries to help you with this such as Conductor.
As for Activities, they are usually overkill for most screens. They don't allow for seamless transitions between screens and prevent sharing UI components across screens (e.g. a Toolbar). Fragments and/or regular views have the added flexibility of being able to use more than one at a time and allows for easy re-use and view composition.
Like I said in my previous comment, the only really good reason to create additional Activities is to provide additional entry points into your app. Otherwise Fragments or Views will suffice and provide greater flexibility and improve the UX in many cases.
As for Activities, they are usually overkill for most screens. They don't allow for seamless transitions between screens and prevent sharing UI components across screens (e.g. a Toolbar). Fragments and/or regular views have the added flexibility of being able to use more than one at a time and allows for easy re-use and view composition.
Like I said in my previous comment, the only really good reason to create additional Activities is to provide additional entry points into your app. Otherwise Fragments or Views will suffice and provide greater flexibility and improve the UX in many cases.