I usually cover foam with a mix of wood glue and cheap paint. Mix in some rubble/grit to give it some texture.
Wooden sticks are usefull for building ladders, barricades, bridges.
Aluminium foil can be used to create the texture of a tree trunk (wrapped around a toilet roll or something for example). I've also used it create a river bank. It's cheap, easy to shape and after you cover it with wood glue, it will hold the shape well.
Egg cartons usually have 1 side which has a texture similar to brick. For example I've used this to cut brick shapes from and glued it to a plastic cilinder to create a tower.
This Youtuber uses a lot of cardboard, so might give you some inspiration: https://www.youtube.com/@BardsCraft