I agree about workbooks. Our daughter loved doing "Homework" at night when she came home. Have them play school with you. They can be the teachers and teach you how to do things..
Have them help you cook. Teach them about measuring. The math on how to double and triple things.
If you see an article in the newspaper you think they will enjoy, cut it out and let them see what words they can recognize or can sound out..
Also have them make story books. They can tell you the story and you print them out and let them copy of of the words and draw pictures to along with their stories.. These make great gifts or thank you notes for friends and family.
If they read, continue to pile on as many books as they can devour.. Ask if you are allowed to check out books in the school library (sometimes at the beginning of the school year they do not check out the books right away). If not take them to your neighborhood library and have then check out and read books each day. Remember to always read the author and illustrators names.. Our daughter can still recite them from her child hood books..
Also make sure they get a ton of physical activity.. They need to run jump hop, walk backwards, peddle bikes, balance on one foot, learn to tumble, cartwheels, jump ropes, bounce balls.. Al of that coordination will hep develop their brains..
Very exciting time