Our 4yo loves doing "experiments":

(1) We put on safety glasses

(2) We wear latex gloves

(3) We write down what we're about to do, what we think is going to happen, and what actually happens

There's a whole genre of "home kitchen experiments for kids" books that we checked out from the library and read through and tried to pick a few.

Several are ambitious because they need some unusual reagents we didn't have in the household. But, you know it hardly matters what experiment you do as long as you're enacting the ritual and having fun doing it :)

Some favorite easy experiments:

* Mixing baking soda and vinegar

* Dissolving shredded red cabbage in boiling water, then using this indicator fluid to measure the relative pH of household stuff like pickle juice, vinegar, baking soda, etc

* Mixing solid carbon dioxide with water, soapy water, food coloring and soapy water (great when we needed to defrost the freezer and had a bunch of leftover dry ice)

* Mixing borax and glue

I have to admit that I did more of this in 2020 and a bit of 2021 when we were indoors a lot and not seeing too many people, and that once I taught the kid to read and once the weather was nice enough to go outside and do unstructured play / climbing, the formal instruction disguised as fun has kind of stalled. Right now he's really into going to the library, checking out books, carrying them to the playground, and reading them there, which I am fine with.

Some other fun things to try:

* Splitting light with a prism (large high-quality glass prisms are under $3 and under $10 on AliExpress, but buy a few as they break). Kids love seeing the rainbow on stuff. I could not quite figure out how to combine a rainbow back into a beam of white light.

* Talking about states of matter (solid/liquid/gas) and building a chart of different states of matter of different substances; also "Why Does the Sun Shine?" and "Why Does the Sun Really Shine?" because our kid loves having the secret knowledge that things can also be a plasma

* Attracting and repelling things with bar magnets; what's magnetic, what's not?

* I've trained my kids to yell "g sin theta!" when they roll down a steep ramp (pretty high compliance rate) and "I'm a pendulum!" when they're on a swing (they now love watching things swing back and forth). This is just for fun. When they're going fast/slow on a slide I've tried to remind them to think about things like how quickly things roll down a steep vs shallow hill, how friction slows them down on a slide and what makes them go faster, whether a longer pendulum goes faster or slower, whether two things of unequal weight take the same amount of time to fall when you drop them … I'm not sure it sticks but it's kind of fun

For math skills I've been working through some of the stuff in https://naturalmath.com/moebius-noodles/. The 4yo is now comfortable enough to start reading this on his own and ask to do experiments, which is nice. The authors argue that kids will get enough natural exposure to certain kinds of math skills (counting things and subitizing groups) that it's worth refocusing them on other fun insights (symmetry, finding like patterns, functions) that don't always get early emphasis.

* The 4yo loves building "function machines" (let's draw a machine that makes things twice as big!) and enjoys making sample inputs and outputs, plus chaining them together.

* My kid also really likes "doing workbooks" (Highlights and Brain Quest make Pre-K workbooks which he loves playing with) and also enjoyed it when I printed out some addition worksheets and showed him how to use a number line to add numbers. He then ... wrote his own worksheets? ("3 + 5 = ____"). OK kid. We also did a bit of work with a number line to visualize addition, then talked about subtraction, but I didn't want to go too far down this road because there's plenty of it coming in school.

It's also worth a visit to a local science museum to see if they have any demos that might be interesting.