For an interesting variation on traditional building blocks, try making your own using natural materials.
To do this you'll need branches of varying sizes. Remember you'll be cutting through these so don't choose anything too thick. Pieces of bark are also very useful and don't forget to include conkers and pine cones as these make great accents to any construction.
You needn't try to make the pieces too regular. In fact, an irregular size and feel of the blocks increases the challenge of building with them, as well as adding a sensory dimension. Nor are you limited to cutting only flat-sided blocks to stack. Thin, finger-like twigs and branches make very effective bridges, and can be leant together like a teepee, to create dens for toy figures.
Depending on your child's age, they can cut some of the thinner branches themselves using a bow saw. Be sure to hold the wood firmly in place with a vice or clamp. Wood from living trees will crack as it dries out so, if possible, use dead branches or leave the wood for a month or two to season.