Well I personally know a physics teacher who does extreme physical experiments using dry ice and liquid nitrogen. One simple experiment is pouring really little amount of liquid nitrogen into a 0.5l coke bottle then wait. We stood away from the bottle (~10m) and let it explode. It is really loud when it bangs, otherwise it's a safe experiment (of course it's done outside and you really shouldn't hold it in your hand).

What's the weak point of a thermos? It's definitely the cap. I think leaving it alone would have just sent the cap flying, it could have been safely done in a park.