My children love to go on nature walks. They each take a bucket that they fill with "treasures". When we get home, I give them a large piece of heavy duty paper or poster board and they each make a collage.
We also have family night. One person is the guest of honor and gets to eat off of a special plate. We order pizza so there is very little clean-up. After dinner, we all get in our pajamas and watch a movie. The guest of honor gets to pick the movie. Everyone gets a turn being the guest of honor, even my husband and I. The kids love it.
With the holidays coming up, we are starting our holiday crafts. I buy craft supplies in bulk so that they can be used year to year. Crafts are fun anytime of the year. Michaels and AC Moore have a great selection.
My children love to bake. We make cookies. I have the older two measure and my younger one pour. They all love helping to use the mixer. The sugar cookies or mini cupcakes are fun because they can decorate them. Your step children may enjoy taking some of the treats home for later.
Going to a local park to explore is always fun and easy to do with the age range you have. My children are 8, 4 and 3 so I know how difficult it can be to reach each of them. When we go to the park we take the fishing rods, buckets, nets and a change of clothes for each of them. We really encourage them to explore, have fun and get dirty.
I hope some of these suggestions are helpful. Good luck.
Barbara