It's all about balance. The children in the story have a bucket and remove things from a rock pool to fill it. Once the bucket is teeming with life, however, the rock pool is dead and there is nothing left to take. The children decide to put the creatures back into the water and enjoy them where they are, rather than trying to possess them.
It all sounds very preachy, but actually, this is one of those lovely life-affirming stories that grown-ups like the message of and children enjoy for the pictures and the rhythmn.
Also, there's a hidden mathmatical element to it - take a bucket, a tray of things and recreate the rock pool. You can have fun with adding and subtraction here, counting, and just general sorting (i.e. "Let's collect the things from outside in the bucket." Or, "Let's collect the blue things.")
I chose, however, to give Daughter a bucket and plonk her outdoors. We went for a long walk, picking up things like conkers and acorns. When we got in, we had a go at preparing the acorns for planting next spring - in a sort of filling-ten-minutes-back-to-the-earth-instead-of-watching-the-telly sort of way. We also used our tree idea to use some of the shells and driftwood, and to tie in with planting the acorns. Because let's face it, months of refridgerated acorns is not a toddler-friendly timescale. Instantaneous trees are awesome too...
There are loads of places you can go from here in terms of further reading or further activities. Collecting and planting seeds is a fairly obvious one, seeing what you can find in rock pools, looking at shells and things under a magnifying glass, looking up the animals in the book on youtube (or better yet - seeing if they feature in Life).
You could also enjoy other books such as the Dr Seuss classic, The Lorax. Again, not preachy, and the rhyme makes it very easy to follow even for tiny people.