My day trip gear packing starts from the premise that I (or somebody in my party) may get injured and/or may have to spend an unplanned night or two in the wilderness without assistance. Most of my day trips, whether from a car or from a base camp, are into an alpine environment so an absence of natural shelter is another assumption built into the plan.

First there is gear to help avoid getting lost or benighted in the 1st place or to help get back on track:

- map of the area
- compass
- GPS
- watch (with altimeter, barometer, compass)
- headlight and extra batteries

Then there is basic sustenance both for the planned trip and extra for the unplanned:

- water bladder
- container of some kind suitable for scooping more water (in Winter add a means of melting water)
- water purification tablets
- a little food (requiring no cooking)

Dealing with environmental hazards:

- Sun glasses
- Sun screen
- lip balm
- Hat (for those lacking hair)
- Insect repellent

Dealing with injuries:

- 1st aid kit (detailed elsewhere but pretty substantial; at a very minimum sterile pads, roll of gauze, anti-bacterial ointment, tape, disinfecting wipes)

Extra clothes for whatever might happen:

- rain shell (top & bottom)
- insulating layer(s)
- toque
- gloves

Shelter and related gear for camping out:

- 2 large, orange garbage bags
- space blanket
- para cord
- lighter and a candle
- knife

Stuff to help SAR find you:

- whistle
- orange garbage bags (see above)
- fire starter (see above)
- signal flairs (used with bear banger launcher)
- mirror (part of my compass)
- cell phone (have it along anyhow since I'm not going to leave it in the car to be stolen)

Other useful or nice stuff to have:

- multi-tool with scissors and pliers
- TP
- more cord
- monocular
- camera
- duct tape!

Seems like a lot but it can all easily fit in a 20 l. pack at a reasonable weight.

Edit: Last trip with all of the above in my Deuter Futura 28 AC Daypack (about 3 lbs.) total weight including 2 l. water (4 lbs.) was 15 lbs. So the "dry weight" of the gear and food in the pack was about 8 lbs. The insulating layer was a down jacket. A night or 2 out would not have been fun but absolutely no threat to survival.