Place at least two magnifying glasses (I have durable almost unbreakable plastic magnifying cards I found at a book store) at a table along with interesting natural objects such as rocks, sea shells, leaves, sticks, pine cones whatever you can find that is too large to be swallowed, clean and free of small splinters that may cause problems for their little fingers. You could just as easily place the magnifying glasses as a table with man made objects such as car and truck toys, crayons and paper ect...use your imagination on this. Almost any object the kids would normally manipulate during their day takes on a whole new perspective when viewed a little closer.
Take this activity a step further with olderd children and invest in a microscope or two, they sell them on line and in stores for under $25 and they come with pre made slides of all kinds of interesting things. This is a great science activity that will make for hours of fun and learning.