To be honest Molly, don't try to be too clever, just learn a few words and how to pronounce them properly to show you are making the effort and to appear well mannered.
Hello, goodbye, thank you etc...
Think realistically how you would receive someone coming up to you and randomly remarking on something random and obviously not being able to understand any real response you could give them.
Maybe you could learn a little stock spiel which doesn't need much response (in French anyway).
Oh, another one to watch is the weather. Be very careful with expressing you are hot or cold!
One thing you are likely to find, depending on where you go, is that their English is far better than your French. Languages tend to be taught from a much earlier age over there, taught to a greater degree and continued for longer under their education system.
They also come across a lot of music, TV, film etc... in English. When I worked over there I remember they used to have The A Team and Walker The Texas Ranger on every week day night after the french teen soap type programmes.
It doesn't always work though. When I was there helping teach English in a secondary school, a song came out called Banana Split (classic video!) but they sang it as in:
ba as in back
na as in nappy
na as in nappy
Suffice it to say we encountered a fair amount of pronounciation issues from that.
Word to the wise correcting the French does not tend to be very well received!