In my first placement, I gave my class a book to add to the library, and I had a little note for each kid in their mailboxes... something short and sweet that I had typed up for the whole class, and then I added a little note to each one that I wrote about something special about them. They loved it. For my cooperating teacher, I gave her a figurine of Snoopy taking a nap since she loves Snoopy and we always joked about needing a nap.
My 2nd placement was in England, and I just gave the class a book about the US (I read it to them on my first day there, so it was even more psecial) and some candy. My cooperating teacher got a gift certificate to a specialty store she'd been wanting ot go to but had never been.
(I brought my CT and my class stuff from England when I came back to visit).
I didn't do anything for the parents either time because I wasn't very involved with them in either placement.