Yeah, second that. See if there's a theme and that'll help focus your thoughts.
Obviously you have sites like greek101.com which have a bunch of T-shirt designs which *might* give some inspiration. Er a bunch of other posts on GC in the past have suggested some options too (posters name in italics) cut and pasted a bunch here:
Betarulz!: Some Follow Trends, Greeks create Legends
Shark In Skirt: In life there are leaders and there are followers. Note that you're reading the back of my shirt.
also liked Delta Tau Delta's (this is so corny and cute): Brothers with Different Mothers.
ThetaGrrl: This year, the front says "One Love, Greek Life" and on the back there is a pic of Bob Marley and it says "Let's Get Together and We'll Feel Alright."
Hootie: ... my fiance has a cute t-shirt from a chi-o (SWT maybe?) and the theme on it was Hooters & Harleys. It wasn't for greek week but that shirt has always stood out in my mind as something neat! They even copied the Harley Davidson logo! It's really cute! If themed, could dress like biker babes? slogan "Chi-O's rule the road" or "Chi-O's: Queen of the open road"
or Chi-O Angels theme - slogan "Good girls go to heaven. Bad girls go somewhere else." (that was a Meatloaf song LOL)
ChiOqt: A cute little slogan for Hootie's suggested Chi-O angels would be something like "We're only angels 51 weeks out of the year!" We used that for our Greek Week shirts last year, it was really cute!
SilverTurtle and
zsbradley: I have a friend who had a Greek Week t-shirt. They used the Sesamee Street sign, only it had the name of their "Greek" street, (or maybe it just said "Greek Street"? i don't remember!) and on the back "these are the people in your neighborhood" and had all of the chapters listed.
But agree with docetboy, work out if you're gonna do a theme.