Find a bin or pan with tall sides. Grab some shaving cream, baking soda and vinegar.
Mix the shaving cream and baking soda (I buy a huge bag of it as Sam's Club) until the baking soda is able to make a ball in your hand and not too wet. I just put about a cup and a half of baking soda and then sprayed some shaving cream on top of that. If it is too wet add more baking soda. Then go get some monster trucks and cars.
Then I put vinegar into a squeeze bottle and it did this:
All kinds of bubbling action! The 3 and 5 year old played with this for quite a while. It is not a neat activity.
But they were due for a bath anyway!
This was pretty messy but the clean up was not that bad and the kids really loved it. I actually liked playing with it too. It is pretty cool, kind of like moon sand.