One of the basic implements you need at a combat-oriented LARP is a foam weapon. Whether you're a new player in the Osgiliath group, or you want a cosplay weapon that is suitable for spur-of-the-moment mock combats at a convention, the construction is the same.
Begin with a length of 3/4" PVC piping. Wrap the pipe in 5/8" pipe insulation, leaving space for a comfortable grip and extending the insulation at least a half inch beyond the end of the pipe. Don't forget to put insulation on the pommel. You don't want any place where the hard edge of the pipe can strike you or your opponent.
Roll a bit of the insulation and fit it inside either end of the insulation. Then cut a piece of open-cell foam, the kind sold at craft stores for couch cushions to add another inch to the end of the weapon. This will be the weapon's thrusting tip. Attach the tip with a bit of tape.
Once the basic weapon has been constructed, cover it with duct tape. This can be done with the basic dark silver tape, but most hardware stores and craft stores carry a wide varieties of colors, including a shiny silver that works perfectly for mock blades. Run strips of tape lengthwise, rather than in a spiral around the weapon. Wrap a bit of extra around the edges of the grip and the ends of the weapon, and you'll be ready to go.