My favourite bilingual situations is seeing a child "complain/whine" to their grandparent in English while the grandparent tries to soothe the child in French. Both understanding each other perfectly.
Something that blew me away once was this one kid at church who spoke with a British accent--I believe he lived in Whales before moving to Canada. A week later I heard this same child speak with a perfect Québécois accent. I later found out his mom was from Montreal.