I was thinking about the type of activity that ds likes best (he is 4) and most of it is what I would call heuristic play. He likes playing with sand, water, mud, stones, sticks, grass cuttings, etc etc etc. He does strange and complicated things with these in a very focused and determined fashion. I think they are vital to him learning about the world and making sense of it.
I think the kind of play that he does in our garden, in the park, at the beach and in the woods is fantastic, organic play and is a seamlessly natural part of our life as Fillyjonk was talking about earlier. I think I get a bit more of what you were saying now, Filly. Ds is totally capable of making his own opportunities for play now, and has the materials available to do so, and I think these experiences are in a way 'better' - more 'real' and more fully experienced - than more contrived activities I might arrange for him to have - for instance playing with water in the (really excellent) basement at the Science Museum.
However he has only lately been capable of collecting his own materials for this play without my intervention (will now insist on bringing home bucket of special pebbles rather than me saying "ooh, shall we take these home and you can play with them?"), and we are lucky in that we a) have a garden b) are close to a park c) have access to countryside and beach. I know there are lots of children who don't have those, and I think for those children, to have this kind of play arranged for them artificially, by adults at a nursery or play centre, would be much much better than to never have access to this kind of play. I think that's comparable with what we were talking about earlier, with regard to the pre-made baskets v. letting your child forage for their own materials.