We had the big sis make animal masks out of posterboard. You would wear your mask and the little sis would be given a matching mask. Then she would find you. We also did this at our town's zoo , which was free to get in. I realize other zoos are not free!
They have a variety of masks at
www.orientexpress.com. They are foam and very cheap in quantity. Then you wouldn't have to make them. We moms buy them for party favors a lot!
Another thing we did was (we had 30 new members so imagine!) the big sis got a skein of yarn in a certain color, put it all over the room ( which was intertwined with all the other big sis's yarn- their own special color). Lil sis' would get one end and try to get to the other end- their big sis! We did this for my group and I really liked it. In fact, although the mask idea is great, I didn't like it and wished that we had done the yarn idea.