As others have commented, it would help a lot if you were a bit more specific about what kinds of experiments you'd be comfortable with, and where and to what audience you're performing it.
For instance, I've always found the ammonium dichromate volcano experiment pretty nice and impressive, but it does involve fire and produce noxious (and carcinogenic) chemical smoke, both of which can be dangerous if proper precautions are not taken. Also, you can't just buy ammonium dichromate from the grocery store, although your chemistry teacher may well have (or be able to obtain) some.
On the other hand, if you'd prefer something easy, safe and doable using common household materials, here's a simple "magic trick" based on natural pH indicator dyes that I remember impressing my 9-year-old classmates (and at least some of their parents) with a long time ago:
- one red cabbage (you don't actually need a whole cabbage, but it's pretty cheap, and you can always eat the rest)
- a few teaspoons of clear vinegar
- a few teaspoons of baking soda
- a glass bottle, jug, pitcher or similar transparent serving container
- two drinking glasses
Chop up some red cabbage leaves and boil them in a few cups of water to leach out the anthocyanin dye from the cabbage. Once the water has turned a deep shade of purple, strain out the leaves and let the water cool. (If the water turns reddish or bluish, add some soda or vinegar to adjust the hue.) You can prepare the "cabbage juice" a few days in advance, and store it in the fridge until needed.
Before the experiment, pour out about half a glass of the cabbage juice, and slowly add just enough vinegar to turn it clearly red. Note the amount of vinegar needed for this. Then slowly add enough baking soda to turn it purple again, also noting the amount. Just to check, double the amount of soda and observe the juice turning blue.
To set up the experiment, pour the previously noted amount of vinegar into one glass, and the previously noted amount of baking soda (dissolved in a small amount of water) into the other. The glasses should still look empty from a distance. Fill the bottle / pitcher with a glass or two of the cabbage juice.
Show the audience the bottle / pitcher of juice, explaining that it's magic purple fruit juice made by mixing red and blue juice. Explain that the reason it's magic is that you can pour out the red and the blue juice separately, which you demonstrate by pouring half a glass of juice into each of the glasses. If you measured the correct amount of vinegar and soda into the glasses, the juice in them should turn red and blue respectively. You can then pour the juice from one glass into the other, turning it purple again.
Now, at this point, since you're doing this as part of a chemistry demonstration, it should be obvious to the audience that there's a chemical reaction involved. If they're familiar with pH indicators from prior lessons, they may even guess what kind of chemistry is involved. That's not really what makes this trick impressive.
The impressive bit, rather, is that at this point you can do something you'd normally never do with a chemistry experiment — you can casually pick up the glass of "magic juice" and drink it. It may not taste particularly good, but, since the only things that went into it were baking soda, vinegar and red cabbage, it's perfectly safe to consume.
Do remember to explain what the juice really is, before somebody goes and calls an ambulance. This can also serve as a good introduction for a further discussion of the kinds of "everyday chemistry" involved in cooking and other household activities.