If you're sure they're not going to blow on you (take some sick-sacs anyway), then set up staight & level at 1000-1500 agl or more, trimmed and at a low cruise power setting. have your passenger put something small like a set of car keys in the palm of their open hand. Then raise the nose smoothly to 15-20° above horizon. Then smoothly push over until you feel light in your belts and the keys rise off the passenger's hand.
Don't get carried away and get into negative g's. The engine will probably loose fuel supply and miss if it's carbureted. Also, the engine may loose oil pressure momentarily if not set up for inverted flight.
If you ease into it nice and smoothly, the zero g's are more fun than scarry. Rough or erratic flying will only scare people not accustomed to it. Tell the people onboard about the engine sputtering too. It may happen if you get caried away or hold negative too long.