Hey mxxuang, why don't you ask your (girl) friend to dress up like Tomb Raider's Lara Croft, and get a pair of silver desert eagle pistol for her, hehe. I bet she will get a big 'WHOA!' instantly when she step in. 

Well, if you're gonna appear in public, noisemakers (yeah, those things that makes RATATAT sound) or rubber dart guns would do just fine. Seriously, it's better to get those than getting into trouble if you just wanna do a costume party/cosplay. If you don't like the colors of the noisemaker/dartgun/whichever, use gap sealer to cover the screw hole and spraypaint it. Paint the tip orange if you want for extra safety while in public.