Linux, Windows, or *BSD. Make it your learning machine. Grasp Networking, Application, Security, etc
Keep your members updated. Implement a Status Page. E.g. Cachet, Stashboard, Staytus, etc
Implement a Relational Database Management System (RDBMS) server. E.g. Percona, PostgreSQL, etc
Configure and tinker with Operating system–level virtualizations, such as: Docker, LXC, OpenVZ, etc
Privacy matters. Install a Virtual Private Network (VPN) using OpenVPN, tinc or SoftEther software
Donate to IBM World Community Grid. Help scientists solve the World's biggest problems in health
Write or install a Proxy script that can be used for your personal-use or enable access to the World
Use it as a backup server using rsync to keep it simple. Remember, your backups need backup
Build or use it as a bridge gateway/connector for communication purposes, e.g. Slack to Telegram
Add the server(s) to a Dynamic Name System (DNS) cluster; a bunch of NS that share records
Help advance research in medicine, clean energy, and materials science with "Rosetta@home"
Deploy and run as a gaming server. E.g: Minecraft, Team Fortress, Counter Strike, etc
Implement Bitcoin "Full node"; bitcoin mining. Read the hosting providers TOS and AUP first
Install and utilize a P2P relay file synchronization application such as: Syncthing or Seafile
Sponsor the machine as a mirror for LineageOS (Android Distribution) project
Sponsor a Dedicated Server to Freenode or OFTC Internet Relay Chat network
Install and utilize an Internet Relay Chat (IRC) bouncer/proxy such as: ZNC or shroudBNC
Install a cloud storage for personal or private use such as: NextCloud, OwnCloud, Pydio, etc
$ rm -rf / #don't give up!