When my kids were young, I read them many, many books. When they got old enough, they read to me many, many books.Now they are avid book readers. They played hide and seek with their Dad, they had the best fun and I love remembering. In the summer we took them on trips to museums and we had lots of picnics in the park so afterward they could slide and swing and just plain run around. Even better if there was a stream nearby, they could wade in the water and throw rocks. They also kept busy playing games with each other and it's a good idea to have something like a "playhouse" they can call their own to go outside and spend some time. And of course, we watched movies with them and still have many favorites we're hanging onto, like Jungle Book, Winnie the Pooh, Dennis the Menace, etc.
Special projects, art or others, are exciting for kids and there are many websites to give suggestions. Mom used to make taffy just so us kids could have fun pulling it. Also fun is making popcorn balls, and homemade play dough so they can become artistic and use their little minds well that way thinking of what to make with it.
Anyway, just some suggestions and ideas. Good luck and enjoy your kids while they're still young. Mine are grown. I loved those days, always will.