Yeah, you can do a lot with just some very basic materials and some ingenuity. Cardboard, foamboard, balsa & other woods (wooden stirrers from a coffee place are a good source for some free wood), aluminium or copper tubing, scrap wire, Evergreen strip, plasticard, metal foil (can get this free from food packing), Polyfilla/spackle, all have their uses. Even paper and normal card stock has its uses esp. at small scales where the scale thickness works better.
You can also sculpt paving, small sections of wall or areas of exposed stonework and a lot of smaller parts and details with any epoxy putty which you may already have for filling.
Einion