Our family currently has a membership to the Phoenix Zoo, the Desert Botanical Garden, and the Arizona Science Center. Each of these memberships also gives us free admission to hundreds of other zoos, aquariums, arboretums, museums, planetariums, and science centers around the country and around the world.
Each membership only cost about the same as it would be for our whole entire family to eat out at a restaurant once, but it lasts much longer and offers more variety and fun.
In addition, many places offer discounted membership for homeschooling families. Educational field trips should be a staple of any home school. If you have already purchased the membership you will want to get your money's worth by using it a lot, rather than using the cost of admission as an excuse for staying home.