The Attackkids recommend these from their (still continuing) childhoods with me:
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The button experiment - put different coloured buttons in the snow on a sunny day. Come back in a few hours, and the dark buttons have absorbed more energy and sunk deeper in the snow
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The grain elevator demonstration - Grain elevators are at risk of explosion, because the fine grain dust in the air creates an air-fuel mixture that can easily explode. This can be modeled by putting a straw through the side of a coffee can at the bottom, then covering the straw with fine flour. You put a candle or lit zippo in the can for an igniter, the puff through the straw, creating a cloud of flour. Poof, a foot tall flame leaps out of the can as the flour/air mixture hits the flame. This should be even better with non dairy creamer.
Maybe that second one should wait till she’s older
I’ll have to think on it for a bit (I’ve had a migraine all day), but I teach preschool with 4-year-olds, and we do a science experiment every week.
Cleaning pennies: You need about a cup of vinegar and a spoonful or two of table salt. Find some old pennies, let them sit in the vinegar/salt solution for a few minutes. Let her scoop them out and clean them with an old toothbrush. Shiney pennies!
Making chalk: Mix some Plaster of Paris (third of a cup of so) and a spoonful of powdered tempera paint in a Ziploc bag. Add a little warm water, enough to mix it well, but not too thin. It should still be fairly thick. It’s easy to mix in the baggie, as long as she doesn’t dig at it. Let it harden overnight, and she’ll have chalk! You can leave out the paint, and the chalk will be white, but making different colors is more fun.
Magents: Find a shallow cardboard box, and line it with a piece of paper. Add a small spoonful of paint and a magnet. It’s better if the paint is in a dribbled line, rather then a blob. Run a second magnet underneath the box, making the magnet in the box move through the paint. We’ve had some pretty amazing pictures come from this. The box needs to be thin cardboard, like a shirt box.
Rainbows: You’ll need vinegar and oil. Add some food coloring to each (different colors for the oil and vinegar) and carefully pour them into a tall clear container. A tall drinking glass works. The layers will stay separate, and the colors make it pretty.
Flowers or butterflies: Use a coffee filter and markers. Let her color on the coffee filter with bright or vibrant colors. It works best if she covers the whole thing. Hang it up (outside it good), and spray it with water. We use squirt bottles. The colors will run and mix. We then make them into either flowers (gather from the center with a pipe cleaner) or butterflies (gather across the middle with an old-fashioned one-piece clothes pin and add pipe cleaner antenna) We use the really big filters at work, but it works just as well with the regular size.
I forgot to mention:
We’ve done the one mentioned by IvoryTowerDenizen and it’s a lot of fun.
For the coffee filter butterflies I mentioned, the markers need to be washable, so they’ll run.
We also make the cornstarch and water mix, and the kids love it. We call it Goop.
Terrarium. Clear glass jar, some soil, water and perennial seeds. Secure the lid, place in area where it gets some sun. Life cycle, condensation, visible roots, etc.