Here is a simple, but funny little activity. Take a bar of Ivory soap (yes, it needs to be ivory) and microwave it. I found the idea here. She says to cut the bar into thirds or quarters. I cut ours in half and it did just fine although it did expand a little bit off of the plate. We did it two times.
The boys had a great time watching the soap expand.
It is hot fresh out of the microwave, but cools off quickly. Then you can touch it. At this point my oldest became bored with it, but my younger two had fun crumbling and playing with the soap.
Ivory soap is pretty inexpensive and it gave us a little entertainment for the day.