Openings against easy opponents

Good day.

I'm going to assist to a tournament this Tuesday in the category for less than 16 years. Many of the opponents know very well how to play, but don't know any kind of opening-defense apart from the well-known ones (Sicilian, Giuoco Piano, Ruy Lopez, Queen's Gambit, etc.) So I was looking if some of you could tell me openings (or variations for the main openings-defenses) that, if the opponent doesn't know what to respond, they will get in big problems.

I had very good results with the Ulvestad variation for the Giuoco Piano, and was looking forward for the Evan's Gambit, will you recommend this last one?

Thanks, Marcos.

How about some modern openings against Queen Pawn openings, such as the King's Indian, Queen's Indian, and Nimzo-Indian Defenses?