I've done all the usuals as PPs have said- pool, cafe, restaurant, beach (mmm, sandy), stationary car etc.
I went to a wedding where DS was meant to go to sleep but never did so I fed several times at the reception - in a dress that only had side zipper access on one side so I was extremely lopsided by the end of the night.
In a taxi once! The cab with the carseat never turned up to get to the airport so I had 4 month old DD on my lap and she was screaming. The male Indian taxi driver basically begged me to feed her! It worked well for everyone, despite feeling extremely dodgy!!