At 5 months kids still need to be stimulated in basic ways. The plush toy is not a bad idea either (stimulates the "touch" sense) but chances are the kid already have enough to drown in if you thrown them all at him.
A brightly colored noise maker will stimulate both visual and audio senses. Plus he will learn that an action (shake) has a consequence (noise), which is more than the little bastard that tried to break my ribs on Sunday ever learned.
Oh, yeah, and if the rattle makes enough noise, he could possible also learn where his parents hide stuff