Several relatively useful items depending on the adventure and the environment:

spare bootlaces
spare thermal socks
empty H2O proof closeable envelope/pouch
rescue throw bag (rescue on the river, ...)
climbing rope
weather station (or watches with weather monitoring)
fire extinguishing spray
pepper spray/bear pepper spray
pocket knife sharpener
binoculars
Morse code chart
ear plugs (when you want maximum loudness from the whistle)
something inflatable (usefull as a float or pillow)
titanium/stainless steel mug/bottle (e.g. for boiling a saline water in order to condensate the water vapour and make a fresh water - fresh water can be then captured by placing a cloth above the mug with the boiling water)