The directions do suggest radish seeds because they are large in size and quick to germinate, but most any seeds should work. Look on the seed packet for the time to germinate and pick seeds that won't take too long. Only put a few seeds in each balloon. This can make a neat science project by having your child keep a record of the plant's development. Have him or her check the balloon at the same time each day and draw a picture to show how many seeds have germinated and how it's growing. Good luck!