Number one top item: brain
If you don't lose your head, you can improvise just about anything.
Skills are weightless. Knots, improvising cordage, firebuilding skill, first aid and so on.

Favorite multi-use items: * Quilt worn around camp for warmth; * Poncho/cape used as hammock fly, general rain protection; *Pack cover used as gear hammock, water carrier, bathtub; *Platypus turned into shower and in first aid for wound irrigation with addition of a perforated cap; *Everclear as stove fuel and for recreation; *Salt as dentifrice as well as condiment; "Backpack as bivouac cover; *Stuff sack (with simple shoulder straps) used as day pack.