"Really? How does your regular grocery store shopping count as a 'field trip'?" I feel like even as homeschoolers we kind of roll our eyes at the thought saying, "Oh yeah (cough, cough) we went to the grocery store as a field trip!" (awkward smile)
Well guess what? All those errands you have to run as a stay-at-home parent DO COUNT...as long as you take the time to make it count. This means instead of frantically dragging your kids through the drug, department or grocery store you're gonna have to SLOW DOWN. Let them be curious and ask questions. Talk to them about the people you see, point out strange food or let them check out an aisle you don't normally walk down.
Of course there are always days when you have to run into the store just to grab X,Y and Z so you can run home to make food for your family. But on normal, non-hectic days don't feel bad about taking your kids to run errands. Aside from some great math, reading and social studies practice there are tons of life skills they can learn and practice.
For ideas on how we homeschool AWAY from home check out these posts: grocery store schooling, homeschool at the docks, at the river, in the woods, at the dump, at a hotel.