What's a good climax for this story?

Two friends go on a trip to Paris but one of them is a backstabing friend and is planning to make her go to jail. They stay at a hotel for awhile then the bad friend gets into an argument and runs out in a huff. The other friend tries to find her. But in the beginning of the story there is a misterios gift with no return adress and the bad friend says don't open it yet. I was thinking bomb, but it seems kinda obvious . Can you help me make a twist or something. Please help