Navidrome for self-hosted music server: https://www.navidrome.org/ (best to pair this with an external portable HDD/SSD for music storage)
Set up syncthing on it and use it for automatic file back-up: https://syncthing.net/ (that external HDD/SSD will come in handy again).
More advanced, should pair with a domain:
Nextcloud for your own personal cloud/calendar/etc: https://nextcloud.com/ - it won't be blazing fast but it will work well, I had it running on a pi for a while.
Simple web hosting - the pi is great for hosting low traffic stuff like blogs etc.
Seconding Syncthing! You don't need a rpi to get started, but it's fantastic having it around as the always-on node you can use to sync multipe devices without them being online at the same time.
For Pihole it's probably overkill. I run mine on Pi Zeros.
My 2 cents:
Simple, no domain required stuff:
More advanced, should pair with a domain:
Seconding Syncthing! You don't need a rpi to get started, but it's fantastic having it around as the always-on node you can use to sync multipe devices without them being online at the same time.
For Pihole it's probably overkill. I run mine on Pi Zeros.