Happy Fun Pundit makes the case that planes should be stocked with defensive items to help the passengers foil a hijacking.
Long armored gloves, wire-mesh vests, and a nightstick are excellent tools for subduing someone with a knife, even in the hands of the untrained, and yet they pose no risk to the aircraft because they are not offensive weapons.
How about the stuff that's already there and dispersed? Reaction time is critical in a situation like this, the hijackers are at their most vulnerable in the first few seconds of the hijack.
Use the seat cushions as armor. They will stop a short knife or a box cutter, and reduce the penetrating ability of a larger knife.
Blankets can be used to entangle a knife-wielding opponent. The Roman gladiators frequently used fishing nets as defensive tools.
The in-seat phones can be yanked out and thrown at a hijacker. It's not likely to put him out of commission, but solid hit will hurt like hell, and its one heck of a distraction while blanket- and cushion-wielding passengers are counterattacking up the aisles, especially since there are a lot of those phones available as weapons.
Smaller items of carry-on luggage can be thrown. A solid underarm toss of a fully-loaded hardshell suitcase is gonna hurt like hell when it hits, and also clog up the aisle making it more difficult for the hijacker to move. This could be handy for helping to contain the hijackers in the rear of the plane while the passengers in the front reclaim the cockpit.