Another thing the kids did while we were at the cottage was build fairy or cricket houses. The found a section of land where crickets played and with a multitude of sticks built mini houses.
This is actually what lead up to building lean-tos and forts that they could actually play in.
To build your own break a whole bunch of sticks to similar lengths. Stick four sticks straight down into the ground in a rectangle or square then lay the the branches around the outside lincoln log style. When finished you can try makeing a roof–Rachel used sticks, Essie used leaves. Honestly you can do this however you like and let the kids see what works for them. I only made a few suggestions–they spent several hours working on them without my input at all.