Totally agree re the clam shell........ours is still going strong.....sand in winter and water in summer.

We also have a little pop up tend (cost about $10 from a variety shop) and it is eternally popular. Some days it is a train, other days it is a cubby......it allows them to use their imaginations, and also houses all sorts of "stuff")

We have ride on trikes, which annoy me as they often get dragged inside, but the kids love them.

Olivia & Charlie both have a gardening set which gets a lot of use (little watering can and mini trowels etc)

For Christmas DH we organised them a cubby house, and we will add bits to that as we go.......

And we also have a "tunnel" thing.........it was a hand me down from a friend.......and all of the children still love it. It is about 3 m long, made from blue tarp that coils into itself, and they use it to link the cubby to teh tend, or just to roll round in......Lexie in particular loved it when she was crawling....