Well you definately need to do a philanthropy drive of some sort - and if you don't currently have one - make it a competition. Can food drive or something of that nature. Heck, make it a campus-wide event that PR's the Greek Community to the rest of the campus by encouraging them to compete with you prior to Greek Week.
Some other ideas:
Limbo Competition (I'm the two time Greek Week champ and I'd be remiss if I didn't mention that!)
Turkey Bowl - not your normal bowling event - frozen turkeys are used to mow down the pins - done outside.
Bike Race We used our Fraternity Circle for this.
Mud Volley Ball Who could resist this campus tradition?!
Greek Sing We didn't do what most normal Universities do we did a skit/song-rewrite for ours. We were given our song two nights before the competition and were required to rewrite the song and incorporate acting/skits with it.
Faculty Recognition Hold a reception for favorite faculty members. Nothing like campus PR to faculty and staff. Each group selects a specific number of honorees and a small number of each GLO to attend.
Izzy Dizzy Slip and Slide Another personal favoite. lol Put a baseball bat to your forehead and keep to the ground - go 10 times around one way, 10 the other and run 5 yards to the slip and slide and see who makes it the furthest. I got lots of laughs out of this one.
Variety Show If they other thing I mentioned didn't work - you could do a variety show where there are different entry groups and each GLO could enter as many as they want - Battle of the Bands, Dance, etc.
I'll ponder more ideas