Here's a great idea that I brought to our Panhellenic Counsil!
A Chi Omega chapter at a nearby University does something called "Send a Crush".
What you do is you have a table in the student union or you send out a letter to all the greeks on campus. At the table you have forms where a person can purchase CRUSH soda (or any type of soda) for a dollar and specify who they want it delivered to. Then when you deliver the soda (either around campus or at individual fraternity/sorority meetings) you give them the option of paying another dollar to find out who their secret CRUSH was

We couldn't do it because our campus won't let us sell pop (has to be baked goods)

. But another sorority adapted the idea for Valentines day and simply had a sheet of paper where you could write a secret note to someone for a buck and then pay a buck to find out who sent it to you

.
So you could do that for May Day or just as a Secret Crush event in general.
Another idea is selling phone cards. I haven't done that since High School so unfortunately I can't point you in the right direction of a company, but I'm sure you could do a little research and find something.
Other than that you could do a Bowl-A-Thon with another fraternity or sorority, a silent auction, sell May-Day baskets (I've got a great way to make them, email me for instructions)...those are my ideas. Hope that helps!
Love In Chi Omega,
Hootie