We use the plant trays I mentioned in this post to make our land forms. Clay doesn't work well for us as it is too hard for Finn to shape. We use brown play dough instead which is really easy to use. At the moment Finn and I fill the bottom of one tray with it, then I make a faint "island" mark for him to cut through with a knife and we transfer it to the other tray. He loves pouring the water to discover his island and lake! Then we have to add wildlife and vegetation (weeds from the garden). He really loves doing this!
He spent a lot of time pretend playing with the animals on his land forms and then came up with the idea of making an island and lake with Lego! I helped him make them and here is the result:
I'd seen land forms done with rocks, sand, clay, but never with Lego before!