Group Work in English Class

With my brightest classes, I'm trying to maximize the time I give them in small groups where they can practice English. In a large class of 60-80 students (I counted heads this week and found that I had 80 students in one class), individual students have very little "talk time". The small groups provide that time, although it gets quite noisy. Since the Chinese have much interest in the culture of America and other countries in the west, I plan to build some lessons this term around activities of American high school students. I started this week with a presentation on "field trips" made by American students. I prepared a slide show of photos with students visiting the White House, going to the museum and aquarium, making cycling trips and going on visits to national parks and nature sites where they learn more about science. First I had to pre-teach new vocabulary which always involves a lot of creativity. The students loved the presentation and laughed and "wow"-ed all the way through the slides, but it seems that I lost them when I moved them into groups of eight to dream about and plan -- all in English -- a class trip they'd like to take. I'm not sure if they just weren't able to reflect on the presentation and use critical thinking skills to see that I was asking them to plan their own trip, or was it simply that they didn't have the language to understand the task. It worked well in some classes and not so well in others. I'll do some revision next week with more simple language to see if it works. These small groups will be an opportunity for the students to improve their language skills, so I want to stick to the strategy if I can get it to work. Standing in front of the class and merely having them repeat what I say or read from the board won't build fluency.