I believe children should be allowed to choose to not participate in group time. Keep them brief and try to find ways to make them more exciting and inviting so children will choose to participate. Video tape your group time in order to better self evaluate and improve. One of the teacher's most difficult task is self observation. Only a special, high quality, participatory field trip would be worth the wasted time, waiting, lining up, and riding on a bus for 30 - 60 minutes.
Many field trips have no meaning to children.
I read where a group of four year olds were taken to meet the hostages that were coming home some years ago. I have a few questions for you and everyone else out there.