1) Move your production website to it (or create one).
2) You're going to need DNS. he.net provides it free, but you're going to do it yourself from this box.
3) Don't forget your backups. While you're at it, better do off-site backups too.
4) You know what would be cool? If your website had subdomains. A dev site for your production site would be a good thing.
5) Many organizations use Linux packages for their mail servers. Can you? Multiple accounts, too.
6) Security! Make the firewall work. Bonus points if you can do SELinux.
7) Oh, and this is the real kicker--can you do this only using the books at your disposal, the man pages, official software documentation, and the RFCs? In other words: no IRC, no forums, no blogs. Official sources only.