You have two choices, meaningful names (e.g. "web server," "primary database") or meaningless names (e.g. "server1," "server2," "James," "Bob," "Sarah," etc).
A lot of people recommend AGAINST meaningful names since attackers can use them to better target their attacks (although arguably it is normally trivial to figure out what is what anyway).
If you're going to have meaningless names then I just leave the default (e.g. rack123.server456) since no matter if you use planets, characters, or toys you'll still ultimately end up using its function to describe it or its IP address anyway.
Realistically your servers and their purpose should be tracked internally via some kind of system (be it Google Apps Spreadsheet, intranet page, resource tracking system, or similar). Which makes the actual host name meaningless.
I hate meaningless server names. It's cute for the first 3 but once you're over 10 you won't remember the difference between the 'obsidian' and 'sovite' servers. Which one is production and which one is dev test again?
At least with a logical naming scheme, you'll have a reasonable idea whether you're even on the right server or not.
For systems consisting of "many" (5+) servers all providing more or less the same functionality, I usually just go with numbering them (s1 .. sN).
For fewer and more specialised machines I'll think of something I like, my private machines are named after the deities of the Cthulhu Mythos (azathoth, cthulhu, shub, hastur, lloygor, nyarlathotep), while some of our Office servers get their names based on dinosaurs and other prehistoric animals.
I name all my computers after Tolkien characters, I have a friend who names all of his after breakfast foods, my alma mater named their servers after Norse gods, all the computers where my wife worked were named after sci-fi starships. If you can't find a theme you all like, naming them something boring like `file` `print` `db`, etc. would probably work too.
If you're looking for something a little less nerdy than some of those suggestions, why not names of streets or neighbourhoods in your city? The main key is that you want a theme with some headroom (you don't want to run out of names at five or six), and you want names that are fairly short and aren't easily misspelled.