Are field trips important? I'd say "yes!" Not just because they are fun for the kids, but they are fun for the moms or parents, in other words, they are fun for the whole family, AND they give you a day away from the books and the usual schedule.
How many field trips should we take each year? Well, that's entirely up to you. I know people who do a field trip every week or two, and then there are those who do one a month. When we were part of a homeschool small (cell) group, we did a field trip once a month. Then there are those who do one, two or three a year. It's really a matter of personal preference, and whether or not you like them to tie into your studies, and whether or not they cost money and you can afford them. We have been fortunate enough to find quite a few in our area that didn't cost us anything, some that cost a little bit, and then those ones that we save as extra special because we have to plan them into the budget.
Some of the free ones we have enjoyed have been trips to a local grocery store, a pet store with a touch tank (That Fish Place/That Pet Place), Cabela's, our local farm show, a nearby town that has enough historical buildings that you can get a little map that you can walk through the town on your own and read about the buildings and their chocolate factory is free to go into and watch the ladies making chocolate products, but of course, you'll want some money to spend in their store because who can resist chocolate?!
We've also had a free tour of our state capitol building.
Some of the field trips that cost a little bit of money have been museums, historical buildings like the homes of past presidents or other famous historical figures and some small, local zoos, a local dairy.
Some of the more expensive ones that we have had to plan into the budget have been a trip to New York City to visit the Statue of Liberty and Ellis Island and a couple of other things.
Then, of course, there are the family vacations, when we can afford to take them. We always count places we visit on vacation as field trips as they usually have educational value. We rarely just take a vacation to simply relax at the mountains, and never the beach. We love the mountains, though. We don't enjoy the beach.
So there are lots of great options for field trips, and they can be close to home or you can make it a day trip, a weekend event, or a family vacation. They are a part of the fun of learning!