This may seem hard, but it is easier than you think. This is one of my favorite DIYs since you get to pick a fun toy or colorful object to go inside, and then display it in the bathroom. All you need to make this is a mold, clear glycerin soap base, and fragrance (optional) You heat the ingredients in a microwave, and clean up is super simple.
How To Make Soap with Cool Trinkets Inside (EASY)
How To Make Soap With Trinkets Inside