1: Pihole is a great addition to any lan. It is IMHO better kept separate to other service on your LAN because a DHCP/DNS server is absolutely fundamental to your whole IP network and should not be allowed to fail. Also you get local name resolution, both forwards and backwards. Of course there is a home assistant integration, just to keep me on topic. And a proper network setup will always assis home assistant.
2: Get a screen and make a nice control panel for your wall.
3: Make a weather station and integrate it with HA.
4: Put some relays on it and use it as a garage door opener, integrated with HA.
There are so many pi projects out there that if you cannot find anything to do with it, your google skills are failing you :). Most of the projects you will find are open source, so integrating them in some way, via rest, command line sensor or mqtt is, well, non trivial, but easily doable in most cases.
Let us know what you end up with
EDIT: media player, either squeezebox (audio) or kodi (video and audio) - both of which integrate with you know what…