Experiment with cooking and baking, since many of the principles there involve chemical reactions. Try making cookies with less or more sugar, or substituting honey for the sugar to see what difference it makes. Boil water for pasta with and without salt to see which boils faster.
Look at the moon and stars at night. See if you can spot craters on the moon. Talk about how sailors used to use the stars to navigate. Buy a chart of the night sky for your area and see if your children can recognize any constellations. Ask them to draw the locations of the stars, and planets if you can see any, on paper and check the paper again in three months to see how the locations have changed.
Fill a container with three parts baking soda and one part vinegar. Slowly add baking soda by the teaspoonful. Don't add it too quickly or it will bubble over the top of your container. Add a little food coloring, and let it mix in. Then add small objects, such as grape halves or grains of rice. They will fall then rise with the bubbles. The vinegar and baking soda create carbon dioxide, and bubbles form on objects in the water. When there are enough bubbles, the object rises to the top, releases the bubbles and falls again to repeat the process.
Static electricity can be fun to experiment with, especially if you live in an area that gets cold and dry in the winter. You can comb your hair repeatedly, then turn on a faucet so the water runs lightly but steadily. Place the comb near the water and watch it "bend" slightly because of static electricity. Blow up balloons, rub them against your sweater and stick them to the wall. Time how long they stick. Check to see if different materials make the balloons stick more or less time. What happens if you rub the wall with some water?