A day out sailing can be an opportunistic moment, but don’t let that spur of the moment decision compromise everyone’s enjoyment and safety.
Have a checklist to run through to make sure you cover the bases. Here is a number of items you can choose from to create your own checklist.
Before heading to the boat:

• Check the forecast, including predicted wind strength, direction and changes throughout the time away (short and long range forecast),
• Tell someone where you’re going and when you’ll be back,
• Prepare your guests ensuring they are appropriately dressed, and bring what they need, don’t forget seasick prevention,

Before Transporting:

• Check wheels, tyres and trailer wheel bearings, lubrication, looseness, air pressure, including spare,
• All Lights operating, wiring plug
• Trailer hitch, and safety chain,
• Tie-down straps, winch and cable, safety chain(boat).
• Safety gear (prepacked in a watertight bag),
  • flares,
  • V Sheet,
  • Horn or whistle,
  • maps/charts,
  • First aid kit
• Safety gear (stored onboard),
  • Fire extinguisher
  • VHF, UHF,
  • compass, GPS navigation, sounder
• Rigging and sails (condition/serviceability)
• Check Fuel and oil, check battery, test run engine, look for fuel/oil leakages,

Launching from a trailer:

• Drain plugs in and tight,
• Prepare the dock lines and deploy the fenders,
• Brief the passengers and crew about procedure, who does what and when to do it,

On the boat, Before starting off:

• Sniff around the cabin for suspicious fumes,
• Visual inspection around the motor, looking for leaks, loose wiring, corrosion, chaffing
• Double check bilges for fluids or leakages,
• Open sea-cocks, breathers,
• Warm the engine

On the boat:

• Show guests around, stow personal items and gear,
• Show where PFD’s are kept and show how to put them on, include how to exit the boat in an emergency (eg, cross your arms across your chest before jumping overboard, so the PFD doesn’t knock you in the face when it hits the water),
• How to operate the marine head,
• What to do in an emergency,

Just before heading out:

• Turn on Navigation, sounder, VHF and log in (to VMR)
• Ensure you have checked your charts are handy, course plotted, location of key navigation marks on your course,
• Assign roles for each crew member, when leaving the dock, handling lines and fenders, etc,
• Make sure non-crew are seated comfortably out of the way

Add to the above any items particular to your vessel, and laminate it for weather proofing, keep a copy handy for reference.
If your the only person aboard who knows how to sail, then take any opportunity to teach others how to handle the boat and what to do in case of an emergency.