The old woman thing is brilliant! I was thinking of it after seeing a group out one night dressed like that...for their friend's 25th!!! As i'm almost 30 I was slightly offended!!! :)
I've decided to just have a Hawaiin flower garland thing going on, as that's where we're going on Honeymoon, so thought it was a nice nod to it, easy, inexpensive and everyone can still look nice wearing what they want, etc. My friend who was going to Mexico asked everyone to wear a sombrero, etc. My other friend just asked everyone to wear a tiara, then her's was done up with a veil, sash etc.
I know there are lots of nice ideas out there that are funny but I think Brides should be conscious of time, hassle and money restraints of their fellow hens!
Good luck! x