It depends on what else you normally carry, but I'm putting together 4 or 5 kits that are "grab and go" and as idiot-proof as possible. Coghlan makes
these waterproof pouches that hold everything and can be used to carry and sterilize water. The kit contains a sharp, locking blade knife, compass, whistle, poncho, fire starter and tinder, twine, some first aid items, led flashlight and extra batteries, water tablets, mirror, deet/sunscreen, a small multi-tool, a couple of chemical hand-warmers, and currently a "space blanket" emergency bag, but that will be upgraded. I think that's it. We used to carry this stuff in fanny packs, but decided that they were easier to lose or leave behind then throwing one of these kits in each of our packs.
I need to add some duct tape, an amount of folded aluminum foil and a small sewing kit.
Separately for a day hike I carry a map of the area, some food and water, Precip pants and jacket, sunglasses, maybe a fleece, maybe a 3/4 pad, maybe a bivy bag or 1 man tent, camera, cell phone, gps, etc.