Small, travel-size toiletry items are always good to have on hand, especially when stuck in a middle seat between two strangers. Having some tissues, chewies or lollies, lip balms or any other necessary items in your cabin baggage will go a long way if needed.
Medications should never go in checked luggage. Always keep your prescription medication, your prescription and non prescription medicines with you in your cabin luggage. This way, treatment can still be administered in the event of a delay when checked bags can't be readily retrieved from the cargo hold, or if they go missing in transit.
The same goes for car and house keys. It's always best to keep these items in your carry-on luggage in the rare event that your checked baggage doesn't make it on the flight with you.