One of the passenger pegs slips down while riding. It has a pin and clip holding it on. Has anyone tried adding a thin washer to stiffen it up? Or, something else?

 

I thought it would have a thin spring washer in there. If not, that's easy enough to do. Check the other side, too.

 

There should be a 'D' or spring washer between the peg and clevis to help hold it up. Sometimes those flatten out and fail, or the tolerance is just too loose with one washer. You can swap the pin/circlip for a bolt/nylock nut and tighten it down to hold it better, or you could try new, or possibly, two D washers...