Just but it to the beam.

Returns are used when you need to end in the open, not against something.

I agree: butt it to the beam.

You can do whatever you think looks better. If there is just a single beam, I'd think butting into it would look fine. If it is something more like a bunch of rustic, exposed beams, then I'd stop short. A 3rd option would be to treat the beam like a 4th wall to the room and just wrap the crown all the way around.

A crown return is very easy. You just cut is like you would an outside corner and run it straight back to the wall.

Why not continue it on the beam?

Thank you all. I suggested to my wife to continue it on the beam but she voted against it. I shall simply butt it to the beam.
While I have your attention, I have another room where part of the ceiling follows the roof contour, thus the ceiling slants down at this point to a bookshelf wall. I think it would look odd to bring the crown down to the bookshelf which is only ~5' high. Thus I want to end the crown at the ceiling point where the wall/ceiling line begins to descend. How to end at this point? I could put in a return or I could do a more complex cut so that crown met all three surfaces.
Maybe I should repost this since the title is off focus now?

Can you post a photo of the second wall?

We had a couple rooms like that in our old house and the builder just put in returns. Looked fine.

Returns look fine to end the molding 'in air' (as opposed to against something).