I just missed this question:
¿Viajan ustedes ________ placer? Are you travelling for pleasure?
I chose "para" because I read "for pleasure" as indicating purpose. The endorsed answer is "por"; apparently I'm supposed to read "for pleasure" as an originating cause.
Looking at the examples here, the most relevant would seem to be:
Va a estudiar medicina por seguir la tradición familiar.
She is studying medicine because it's family tradition.
It seems to me, you could also use para here, with a different meaning:
Va a estudiar medicina para seguir la tradición familiar.
She is studying medicine to follow the family tradition.
Here it's pretty easy to tell them apart because they differ in English. With
Are you travelling for pleasure?
... I guess I don't even know what "originating cause" means in this example. It's because you wanted to obtain pleasure, right? That seems to me like a purpose.
If the originating cause were a noun ("amor", "los niños"), or a pre-existing state/action ("ser el primo"), then it's pretty easy to see it's an originating cause as opposed to a purpose. But for a verb that seems to relate to future action, I don't know how I would know.