-F-Droid instead of Google Play.
In F-Droid you will find a variety of open-source apps, like the following.

- QKSMS instead of the pre-installed messaging app.
Support for multiple SIMs and MMS messaging.

- Firefox instead of Chrome.
The debate for the perfect browser is out of scope here, but obviously your browser is one of the things you will consider changing.

- NewPipe instead of YouTube.
NewPipe is a front-end for YouTube, basically YouTube with privacy and without ads. It can also play in the background (!) if you like. Since, PeerTube is not always enough, no matter how substantial.

- OpenMultiMaps (based on OpenStreetMap) instead of Google Maps.
Crowd-sourced maps for all countries. Special filters included, like wheelchair accessible places, vegan/vegetarian restaurants and more.

- Carnet instead of Google Keep.
Simple but feature-rich notepad.

- CPU Info to complement pre-installed hardware monitoring apps.
Displays hardware specs, current CPU frequency per core, RAM usage, temperatures and more.

- Orbot to access the Tor network.
This app works mostly as a VPN. No need for access to specific (paid) servers.

- Simple File Manager Pro to view and handle directories.
Google’s Files are designed to show only files saved directly from the user. Finding system folders is not that simple. The Simple File Manager belongs to a series of apps for basic operations by Simple Mobile Tools, most of which are worth trying. The apps require a payment in Google Play, so they are not Free anymore. Yet, you can still install open-source versions via F-Droid, without payments or ads.