Whistle +1,000,000
I’m constantly surprised by how few people will take something that costs virtually nothing, weighs virtually nothing and takes up virtually no space in your pack / pocket, and could easily save your life.
My solo pack for the hills looks something like:
OS Map(s)
Compass
Skills to use the above 2 items
Waterproof
Spare tube
Patches, glue etc.
Pump (wrapped in duck/duct tape)
Zip ties
WHISTLE
Multitool
suncream (if likely to be needed, so not so much this year so far…)
Water
food
Small 1st Aid kit (enough to patch up some bleeding, not enough to perform emergency surgery on myself)
One of those tinfoil blankets that everyone tells me are pretty useless
Chain link
Mobile phone
Cash
I use the same pack for commuting but the whistle lives in there permanently as it weighs virtually nothing, takes up no space and if I don’t keep it anywhere other than in my Camelbak then I can’t find that I’ve forgotten it when I need it.