I dont think it is necessary to carry a full survival pack on a day hike(although I do) But at least carry some essentials like water, and a fire starting device.
... and a knife, a whistle, a signal mirror, and a few LARGE garbage bags for shelter.
A small headlamp would be helpful for night-time signalling and shelter setup.
Doug Ritter's (Equipped.org) Suggestions:
1. Navigation - a mapping GPS w/ spare batteries, or secondarily a map & compass, AND the knowledge to use them together
2. Knife - one-hand opening
3. Fire starter & Tinder
4. Water & water purification
5. Whistle
6. Signal Mirror
7. Duct Tape
8. Shelter - large garbage bags or AMK Heat Sheet
9. Flashlight or Headlamp - LED, lithium battery, with constant-on switch
10. Sunscreen and/or Insect Repellent
11. Personal Locator Beacon
Less Stroud's (Survivorman) Suggestions:
1. Sure-fire way to get a fire
2. Really strong belt knife - "like a good Buck"
3. Orange garbage bag to use as a tarp
4. Something to boil water in
5. Good amount of rope
6. Axe