To save time, we decided to use cardboard boxes and cylinders for our molds rather than building them with wood. We reinforced the boxes with duct tape and cut the cylinders into 2in slices.
We tried wood shavings and sawdust as aggregates but the sawdust seemed to make the concrete crumble.
We also learned not to move the setting concrete too soon, and that some of the bigger objects would need some type of reinforcing.
Perlite proved to work as the best aggregate for our stepping stones.
And the boxes worked really well too - the concrete popped right out!