We're very relaxed home learners...I wouldn't call us radical unschoolers, but we're not particularly schedule or subject bound. I keep a daily journal of what the kids do, and translate that appropriately.
For instance, yesterday we went to the zoo in the morning. I recorded 2 hours of science & 1 hour of social studies for that outing (We talked about what animals eat, their habitats, the geography of S. America & Africa, baby animals, etc). In the afternoon, we had a scout meeting and they learned how to plant & care for a sapling. Guess what? More science
Saturday we went to a play at the local theater (Charlie & the Chocolate Factory) and had a backstage tour w/a local hs group. Recorded as 2 1/2 hours language arts.
Around the house, baking usually gets recorded as math, as do math related games. To be fair, I will say that my oldest does generally do a bit of traditional math work a few days a week b/c she thinks it's fun and requests worksheets, etc., but recent non-book work has included plotting our garden (how many plants in how much space, figuring ratios of plant food, how long our hose is, etc), helping to plan menus (how much pasta do we need to feed the family), and figuring costs at the grocery store (including learning why we make the trip to the health food store for bulk goods).
Nonfiction reading gets grouped into language arts and whatever the subject matter is--a book on ocean life is recorded as science & language arts, one on Chinese history as social studies & language arts. Fiction reading, as well as anything springing from it (plays put on by kids, puppet shows, story writing) all goes down as language arts.
Bike riding, soccer, etc, go down as PE. Arts & craft projects under art.
I think it may be a bit easier as I have a fairly bookish child--she reads happily, requests help finding information on things she's interested in, etc.
The record keeping is a bit of a pain, but really not a problem, if that makes sense.