I'm surprised nobody has brought up the "Ten" Essentials. There are a couple of articles on the home page of this site that you should read:
"Hiking Essentials"
"Day Hiking Gear List"

The day hiking list I cited is about 2 lbs. more than I carry (my daypack is about 10 lbs. with lunch and 2 quarts water), but it's a good guideline. I haven't really analyzed it to see whether the author is carrying more stuff or whether his individual items are heavier. I would doubt the latter since my daypack contains my older, heavier extra clothing and rain gear that I no longer use for backpacking. I do carry enough clothing, plus one of those little mylar blankets, so that I can stay warm, dry and hydrated should I be caught out overnight. I won't be comfortable, but I won't get hypothermia, either. The "extra food" just needs to be an extra snack, preferably something you don't particularly like so you're not tempted to eat it during a normal trip. It takes weeks to starve to death, or longer for those of us with more internal padding. I don't carry a filter for day hikes but take some chlorine dioxide tablets--there are always some in my little "first aid-small essentials" stuff sack. Again, if it's an emergency and you're out of water, drink what's there. You can die of dehydration quite rapidly, while the incubation period for nasties in the water is a week to 10 days, by which time you'll be home and close to medical care.

Yes, you might want your lunch!

Re your cash, etc.: Never leave anything in your car (even if not of any value) or that would provide fodder for identity thieves. I always leave my billfold at home and take a little folding money, drivers license, car insurance card, car registration, health insurance card and a debit or credit card in a sandwich bag in an outside pocket of my pack. If everyone would take care to follow this rule, car clouting at trailheads would cease to be a problem!