I use the following website to get ideas to use with my 3rd grade Brownies Troop for Girl Scouts.
http://www.makingfriends.com/
I don't always buy the supplies from the site but it helps me to think of ideas to use with the girls. (Sometimes I go to a craft store and buy similar stuff.)
Being in Girl Scouts is definitely a way to foster friendships. Would you consider being a leader? You might want to check into it. My troop loves getting together for meetings and other places to earn fun patches.
Girls love having something in common to remember a good time. Make bracelets, necklace, t-shirt, etc... and then they can plan a day to wear them at school together.
Singing and dancing are always "big" at this age. Maybe a karaokee (sp?) machine to hook up to the TV.
Take a group photo and have them design a photo frame to put the photo in and they can have it as a keepsake of their fun day.
Caroling in the neighborhood is fun if it's around Christmas. You may need to print out the lyrics for the girls. Unless you agree to sing the very popular songs that they already know. Come back to the house and enjoy some hot chocolate together.
Playing games from the tv game show called Minute to Win It would be fun. Check out some games/challenges on their website. It's on NBC. My kids love that game show!
Painting nails and spa stuff is fun. Some places like Toys R Us has kits that the girls can make their own facial cleanser. You might want to have 2 or 3 options going on (like stations) so the girls can choose something that they really want to do.
If you really want to foster friendships, I would consider doing this again... maybe once a month (if possible) and each mom would take a turn hosting the "get together". Make a schedule with the other moms.... or something. even if that's not possible, just getting together once may spark a friendship that might never had happen if not for you doing this!
Have fun!
J.