Reflective sunscreen shade for a car windscreen. They are essentially just a piece of light foam with a foil type backing, but they are incredibly useful.
I always take one with me to use under my inflatable mat to add some warmth and puncture protection to the mat, at 50g or so it's a no brainer for me.
I've also used them to make pot cosys and have them behind most of the radiators in my house.
The stainless steel roasting tins are good for making diy pot lids too.
Loads of really useful gear to be had in the various pound shops.