When you are up in casting you start to look for objects that can be used as a mold. One day we went to a store where I saw a bunch of soft rubber balls with knobs on. It struck me suddenly that they could be used as a mold if you cut off the valve and turn them inside out. We also had a mini beach ball lying around at home. It cut it up too!
After filling a ball with concrete I dug a round-bottomed pit in our old sandbox and put the ball in. I left them in the sandbox to rest for a few days before I picked them up and cut a straight line down the side of the ball and then wriggled it out. I could then reuse the balls by taping together the cut again.
Sand molds for children gives great little figures. Bowls and plastic tubs of various sizes will be fine concrete bowls. Some I have decorated with glass beads that I pressed into the concrete.