First of all, here are some suggestions for paying for this trip. Of course, you can always have each child bring in their share. Or you can write grants to pay for the trips or find business sponsors to pay for the trip. There is also the site donorschoose.org that might help you. One time, state senator David Thomas befriended our class and paid for everyone to go see the state legislature in action, then a trip to the zoo including lunch. It was a trip that my kids would remember forever.
A couple of years my administration would not let me take the kids on a field trip so instead I rallied the parents to help. I decided to take a Saturday trip to Carl Sandburg's home in Flat Rock, NC. I invited the parents and kids to meet me in a grocery store parking lot and follow us up there. My whole class attended.
Another year, the families and I met on a Saturday to go to Charlotte, NC to Discovery Place. The kids enjoyed it as well as the parents. On the way home, we grilled hot dogs for lunch.
Another year I wrote a grant about learning how to cut mats and frame pictures. This grant bought supplies and paid for a trip to the local art museum.
If you really want to go on a field trip, talk to lots of people about what you want to do. Someone might have a great idea how to fund it or suggest you talk to someone in particular. When the kids see that you are willing to go through all of this for them, it will really help them see that you care for them. I also used it for a behavior management tool because we never went until the spring. Any student who had been suspended during the year could not go (this was announced the first days of school) and usually that helped them control their behavior.