Are they reading? Here are some activities to challenge them. If they're not reading - no pressure, read stories to them to help build their reading skills (vocabulary, context clues, ask questions, talk about the pictures)
Start building their motor skills now and it will give them a boost in their creativity (right side of their brain), vocabulary (left side of brain) and reading later
Abstract reasoning (left side of the brain) also connects with visualizing - seeing items or pictures that aren't physically in front of you which turns into different levels of understanding and representation. These skills help with math later