My wife asked me whether I could attach an AC adapter to our baby's mobile. I'd seen an article in MAKE about modifying noisy toys, so I knew it was possible. It turns out to be surprisingly easy, provided you (okay, I) don't make some dumb choices along the way.
NOTE The project I describe will void the warranty for whatever toy you modify. The manufacturer (rightly) will not support you or provide you with assistance in doing this. If it works, be happy: If You Can't Open It, You Don't Own It. If it doesn't work, take it as a lesson not to meddle in the affairs of Corporations, for they are Subtle and Quick to Anger.
Step 1: This Instructable is Incomplete
Nevertheless, the descriptions in the Steps are accurate and complete, based on lessons learned the first time. I think this can still be useful to folks.