Here's a really easy one: dissolve sugar in water; evaporate the water and you recover the sugar - that's a physical change. Burn the sugar, however, and you have a chemical change! (That's oxidation. )

Of course changing state - from solid to liquid to gas - those are all physical changes - boiling water into steam, freezing water into ice, for example. Any time you form a new compound with some chemical reaction - that is a chemical change. Put baking soda in vinegar - that is an acid-base reaction.

Here are some things I can think of. Chemists are usually experts in this sort of thing.

Chemical:
Burn something like a match, piece of wood, paper, etc. Sure to be very exciting for 3rd graders. Start it with a magnifying glass and we're talking serious entertainment. Mixing baking soda and vinegar. Also a sure hit.

There are all sorts of interesting products out there that use chemical reactions: chemically activated hot packs, cold packs, and glow lights.

Cook something.
Always involves some sort of chemical reaction. Usually complicated though.

Physical: Melting ice and then boiling the water. Probably pretty boring. In another physical reaction you could dissolve (an amazing amount of) sugar in the boiling water and add lemon juice to make lemonade. Finally, cool down using more ice or...

Dry (carbon dioxide) ice or liquid nitrogen would be much more exciting to play with if you can get some. You can also make ice cream by pouring liquid nitrogen into the ice cream mix.

One neat thing I just found out about. If you dip white Teflon tape (used on pipe threads) in alcohol it changes color from white to clear. Once the alcohol evaporates it changes back to white. I'm not really sure why it does this.

Two part epoxy (we use 5 minute epoxy in the lab). Starts kind as a viscous liquid. Mix it up and very quickly it gets sticky and then hardens.

Back when I was in high school, I made a deal with the teachers and ended up in having to 'teach' a lecture once a week. I was easy at first -- but became quite difficult as I realized the problem of actually communicating to the students...

Easy chemical changes: Simplest sure fire chemical change which incorporates a physical change is baking soda and vinegar
--> release of carbon dioxide and formation of sodium acetate salt. (You can show this if you can quantify the amount of vinegar in the solution and let the product dry out for a couple of days. Acetates grow crystals which are usually hexahydrates -- so should be obviously different from sodium (bi) carbonate.

Another nice putty is made from Elmer's glue and boric acid (commonly used as an eye wash). It becomes very plastic and is no longer adhesive. Essentially also a long-chain polymer (like gluten in bread and in dough). Other nice chemical changes are common colored indicators -- many are poisonous however -- so try boiling red cabbage to make a strong purple dye (ware your clothes) --this dye will turn from purple to green given a pH change-- I recall that vinegar or baking soda dissolved in water were strong enough for a nice color change. (Vinegar is essentially acetic acid so a very low pH, while baking soda solution has a small amount of Sodium Hydroxide which is a strong base. I think the green color is the base phase). Indicators such as the purple dye usually contain conjugated double bonds which absorb different colors as their ion state in solution is changed.

Other good indicators are Methyl Blue (used as a fish tank antiseptic) which changes from blue to yellow and phenolphthalein, which goes from clear to bright red/purple as the solution -->goes toward base. (It is the primary ingredient in ex-lax.) For 3rd graders -- I'd go with cabbage juice.

Another class of easy to demonstrate chemical changes are metal oxidation, such as making a potato battery from a bit of zinc (galvanized nail) and copper (say a penny). You get enough current to run a very small motor --or a meter if you have one. Be sure to show the penny and zinc after you run it for a while to display the galvanic corrosion.

Finally, most dyes used in colored paper are not colorfast when exposed to sunlight --so making a montage, putting it into a sun exposed window for a couple days and then taking it apart will aptly demonstrate sun bleaching of dyes. (This is essentially ultra violet dissociation of the dye molecules).

Hope one or more of these help. As with all sort of these things --please don't let the students into the dyes and or vinegar/soda-- vinegar can sometimes cause mild burns if it soaks into clothing, dye can make permanent stains and soda can cause rashes in kids with very sensitive skin and poor washing habits. However, the color changes are quite striking.

Easy Chemical Changes to Observe:

1. Dissolve Table Salt (Sodium Chloride) in Water, I would call this is a chemical change because when salt dissolves the Sodium and Chlorine atoms become disassociated from one another.

2. Heat sugar up to until it caramelizes. This is a chemical change. Overheating caramelized sugar makes it smoke (and smell bad), another chemical change.

3. When a candle (not the drip less variety) burns the wax undergoes a physical change (melting), but the wick undergoes a chemical change (combustion). Drip less candles are made by coating the outside of an ordinary candle with a wax that melts at a higher temperature. As a result, the inside wax is burned (a chemical change) instead of dripping down the outside of the candle.

4. Mixing baking soda and vinegar (in small quantities) releases a gas. This is another chemical change.

Gluten is a protein (or aggregate of proteins) found in wheat. Kneading the play dough does not cause new proteins to form. I'm guessing that kneading causes these protein molecules to become aligned within the dough. If so, I would not call it a chemical change unless chemical bonds formed between the proteins. I'm really not sure if that happens in this case.