Magnetic number fishing, they add what they catch! As well as addition, develops hand/eye coordination, fine motor--I love this for magnetic letter fishing too! When they catch a letter, they say its name and sound.
This would be a fun idea for kids and a great use for all those toilet paper rolls I can't throw away! Chopsticks or tweezers for fine motor skills, used to pick up and drop pom-poms into color matched toilet paper rolls.