What, if anything can you attach to a beam

If you put a beam across the room you'll feel bad every time you look at it. if you have a flat ceiling, you'll feel proud.

Use joist hangers to put some 2x4s between the ceiling joists so that they pass just above the 2x6 (stuff's going to sag), drill through the top plate from below with a spade bit and screw the 2x6 to the 2x4s remove the wall and top plates patch and plaster the ceiling.

If you can't get into the ceiling to get enough access to fit the hangers you could just lay 2x4s across the top of the ceiling joists and use really long screws, or maybe even come through the roof with threaded rod.

You seem to have posts at the face of the existing wall. If that's the case, you don't really need to attach the beams other than to hold them in place in a superficial manner.

Otherwise, joist hangers or full bearing on framing in the wall. In either case you need suitable backing structure on which to mount the hangers or bear the beams. If you don't have studs in the right locations, look into top-bearing hangers that you can fasten to the top of your wall plates.

The fact that one beam appears to connect at an outside corner may complicate things a bit. You won't have room for a standard hanger. You might be able to attach around the corner by flattening out one leg of the hanger. That also may be a good location to cut into the corner stud cluster and bear directly.