When I was an active, we had a lot of cob events, and we advertised for them by posting signs around the school as well as sticking little flyers in the mailboxes of people we thought may be especially interested (rushees who dropped out, friends of chapter members, etc). Some events we did included:
"Breakfast with Babe" the Rush chair got her grandmother's pancake recipe, and we all stuffed ourselves with pancakes and watched "Babe" on the TV in the chapter room.
"Kappa Kindergarten" with fingerpainting, planting seeds in paper cups, snacking on graham crackers (can you tell we were big on food in my chapter

)
Beach night (you can give it your own clever name). The middle of February in northeast Wisconsin gets pretty dreary, and I thought if I had to see one more COB event advertising "Come drink hot cider by the fire" I was going to barf. SO...I got a beach ball, some beach chairs, pineapple, and virgin pina coladas and we had a beach night, complete with hula hoops. Bathing suits were optional (I wore jeans over my one-piece)
I had a spa night in my very large single when I was a Rho Chi (small school, remember, not as many rushees) for the prospective rushees in my dorm. I played relaxing music and had manicure stuff, facial masks, etc.
Kappa Kamping: smores, caramel apples, storytelling
hope this helps, and good luck with your COB events. My advice as a former rush chair is to keep things simple. Even a night of TV and cookie-baking is fun.