Using different natural materials, create your own pattern (you will need a few of each variety). It could be leaf, stone, leaf, stone or you could use many more things like twigs, daises, leaves, blades of grass etc. You could make your pattern with more than two objects, maybe leaf, stone, daisy, twig, leaf, stone, daisy, twig etc.
What would come next in your sequence?
What would the 10th object be?
What would the 20th object be?
And what about the 100th?
How could you work it out?