Hiya!
Informal is can be so tiring, keep that in mind when you are planning events.
Things we've done in the past:
-pool party @ the campus pool cause it was free. Carefull not to advertise too much all over campus, you could have unwanted guests.
-video game night. This is fun cause guys don't usually share the controllers. bad part- not highly interactive if you aren't careful, but very fun!
-weenie roast.
we were gonna use our fireplace, but theys chool didn't inspect it in time so we did it w/a George forman grill. it was a blast.
-bowling is fun
-must-see-tv or someother type of tv event.
-don't forget PHILANTHROPY!!!! it gives great PR and helps you kill two birds w/one stone. we did valentines cards and gave them to elderly people for an event.
Good luck!!! keep it fun for the chapter but not so much it gets too cliquey.
Tau love