For my CB550F i have a little purple number-four-poolball as a keychain (and for my Honda VT600 V2 i have a blue number-two-ball).
For my other small Hondas i have silver necklace charms, silver earrings and fingerrings i found on the ground, small bearings or italian cornicellos. For my Honda Wave (innova?) i have a red, white and blue bowtie (it was built in Thailand, not Japan), and for my Honda Elite Deluxe with retractable headlight i have a 9mm cartridge (Sarah Connor drives one of these in the beginning of the original 'Terminator' movie).
More important than the attachment is the chain itself, in my opinion: i always use these ball chains used for military 'dog tags', rather than chain rings. they are easier to shove into / pull out of tight riding pant's pockets because they collapse and do not use a lot of space in your pocket. But they still add enough mass and 'feel', and together with a small ornament making a bare key complete.
i started using those ball chains since i saw in an under-dash shot of a car chase in 'Vanishing point', that Kowalski did too. The spare key of his brand new Challenger was very briefly seen dangling off a ball chain from the one stuck in the ignition lock. Unfortunately i cant afford Kowalskis Alpine white Challenger, but i can afford his key chain...