I've put together a nice kit based around one of these...
For scale, it's about the same size as a water bottle pouch.
It's a PLCE medic pouch. It is a bit big for casual belt carry, but it's great for just throwing into a day pack or shoulder bag. It opens out to reveal 3 smaller pouches. One big one at the back, and two smaller pouches at the front. The idea is if you are going out for the day and not expecting to overnight, you can just sling this into whatever day pack you want to use and it's all there if you need it or if you do need/want to stay out overnight.
There is a 2 litre MSR water bladder, folding plastic cup, small FAK with a number of items including 10x pre-made rabbit snares and few plasters etc...
Falkniven F1 with paracord, DC4 sharpening stone, Wenger Scout (excellent) SAK, leatherman P4, more para cord...
milbank bag, space blanket, cheap vaccuum packed poncho...
Comprehensive survival kit, based on the Ritter mini kit but with numerous items added, included water purifying straw, red and white photons, stock cube, mixed spices, salt, and a BCB mini mess tin which I have made a handle...
Better shot of the handle/adapter...