I had a parent ask what she could do to increase her son's finger strength since he had difficulty using his fingers to open lids, button buttons, and with controlling his pencil when writing.  This child is not very compliant with adult directed activities, so she wanted a way to strengthen his fingers that wouldn't necessarily be a "task" presented by an adult.  I suggested giving him a baggy filled with a snack, cereal or toy, for him to open.  Allow time for the child to problem solve through this.  You will be surprised how  difficult a task it is for small fingers.