My understanding is that it's the "Preferred Ride Intensity" stat of a guest that determines what they think about a particular ride, with "I want to go on something more thrilling than ____" coming up if the ride is lower than preferred and "____ looks too intense for me" if it's higher.
Going back to the original question: There are some thrill rides with intensity levels comparable to roller coasters (eg. Rotodrop) that could potentially work in a "no roller coasters" challenge. That still leaves issues with ride availability by scenario and objectives that specifically call for coasters, though.