I really think you are giving yourself less credit than you deserve. Just start fucking around -- build a dhcp server, do s stupid little mail server for your "private office" (aka, lan only). Shit like that. Once you start making shit that is used in the real world your practice tasks, you probably arent going to lear all that much in comparison.

Some good ideas :

Sendmail : Build a simple inter-office mail server.

Squid : Make a caching proxy server. You wont notice any benefit unless ~10+ users are hitting the same sites, but can be nice to know.

Gateway : I have to confess its harder than OpenBSD to pick up on, and I still prefer OpenBSD's pf to iptables .. but it is a BIG one to know.

DHCP server : This one is actually the hardest to do in your mind, but the easiest to do once you have a copy of RH's tutorial. After that, you WILL have most of it memorized. Took me 25 minutes to master 75% of the content.

Samba PDC : Nuff said. Probably one of the top 5 reasons for a linux box in a windows shop.

Apache webserver : Nuff said. Probably one of the top 5 reasons for a linux box in a windows shop.

Thin Client OO.org server : Use a powerful machine to serve out X and OO.org for wimpy clients.