My car keys: (has small Buck knife & opener, LED light good in Emergency or for changing batteries in headlamp in dark & whistle)
food
water
some type of synthetic shirt (depending on season & distance that might be a T-Shirt or two Mirco fleece shirts - usually have both in car)
Except for the shortest of trips in summer (maybe Mt. Tom in MA about 1.5 RT) a rain coat
some type of Space blanket , either the small cheap space blankets or the $25 Bivy bag type
an extra strap or two
cellphone (adding this did not mean I could leave anything I would have brought home)
That would be most of what I might bring on a shorter summer hikes, (under 10 miles, no more than two sub treeline peaks maybe an 1 above treeline not in Presi's, BSP or above 5K in ADK)
Most times I bring poles (part of first aid kit also if I need to splint a sprain/twist with strap & extra shirt) I'm not allergic to anything, when I carried medication , I never checked it enough to keep it from being way expired.
For 10+ mile hikes, tape goes, TP usually goes but in 13+ years I've yet to need it. (with a 4 hour drive to trailheads the rest area's Dunkin donuts, etc... on teh way suffice, I don't need to draw a picture do I? :blink: )
As the season changes or the a Presidential Trip is planned (or both) gloves, fleece pants & tops, rain pants, hats & or balaclavas mean I can take out the ER fanny pack out of the car (because you never know when you may have a couple of hours to kill & your near a peak, state park, forest or rail trail) & the ER 2500 pack stays in the back of the car.
Winter trips I have to plan for, add crampons & snowshoes usually require a 3000 pack