This activity is a useful way of teaching the concept of "pairs" to your preschooler. You and your preschooler will go through magazines and find things that come in pairs, such as shoes, eyes, socks, dice and so on. Have your preschooler cut those pictures out (or you can do it for her), then tear the pictures into two pieces (so you have one item on each picture), mix them up and have your child match the pairs. Provide your child with a piece of white card stock and let her glue the pairs together on it. Later, you can use this as a math manipulative for counting by twos.
This activity is a nice supplement to a unit on kites. You'll need to cut some white construction paper into a diamond shape---like a kite---then add two dollops of paint in primary colors. Give your preschooler a straw (but punch a hole in the top first so she doesn't suck up paint), then have her blow at the paint through the straw, observing how the colors mix together and make other colors on her paper. Talk about how colors mix together to make new colors as you do this. You can also use this as a color-recognition activity by having your preschooler name the new colors that form. If it's a windy day, you can lay a large piece of white paper outside, add the dollops of paint and let the wind blow the paint colors together.
This is a great extension on a water cycle unit, and it's a quick experiment. You'll fill two shallow dishes with water, and then add 2 tbsp. of salt to each. (The more water you have, the longer it will take to see results, so you don't need much at all---maybe about 1/4 cup in each dish.) You can let your child stir the water in each cup until the salt dissolves. You'll tell your child that you're going to leave one cup in the sun (or a windowsill that gets sun) and another in a cool place, such as a closet or his bedroom. If you've studied the water cycle, explain that the water is going to evaporate (or "go away") but that water evaporates at different rates. You can tell him that you think the water in the sun will "go away" faster than the water in the cooler area and ask for his prediction. After three hours, check both containers. The water in the sun should have evaporated completely, leaving behind salt crystals (if it hasn't, wait a bit longer) while the water in the cooler location won't have changed much.
Another way to do this is to place plastic wrap over one of the dishes (make sure you use warm water if you do it this way), and then check on the plastic wrap after an hour. You can remove the wrap and let your preschooler taste the water droplets that are on the plastic wrap and compare that to a drop of the water in the dish. The water on the plastic wrap will taste fine, since water, not salt, evaporates, whereas the water in the dish will be very salty.