Depending on setting you can find useful bits in Poundland etc to either convert or scratchbuild vehicles. Interesting shaped bottle-tops (perfume bottles or Airwick type room fragrancers are usually good) can make interesting VSF equipment/installations.
Plastic dinosaurs (or out-of-scale animals) can be handy for Lost Worlds expeditions.
At this time of year look closely at cheap plastic decorations (a month ago would have recommended looking at Halloween stuff) for scratchbuilding material. Some big tree baubles are ideal for hot air balloons.
Also check the charity shops, often have rummage boxes of cheap tatty toys that can be cannibalised.
Some boardgames can be useful, there are quite a few with figures etc in that can be re-purposed.