Every homeschooler, no matter style or philosophy, will be on the receiving end of this question more than one time. It is a question that can be asked out of genuine interest. It's also a question that can be asked with a tinge of disgust, in which case it becomes the most annoying question in the world. In this instance, I try to smile and be pleasant, give a short and succinct answer, and move about my business. It never helps to be sarcastic (though I have been at times) and it's typically better to be brief (the exception being with a new or potential homeschool parent).
As a person who has received this question and understand how annoying it can be, I admit to wondering it about other homeschoolers at times. I almost always intend the silent question to come from a place of genuine interest (though there are situations where it is thought, I admit, with a "tinge of disgust," - yes, I'm human and have negative thoughts from time to time). Despite the good intentions behind my wondering, however, I generally refrain from asking the forbidden query. I have asked the more acceptable questions such as, "Do you do fill-in-the-blank activities? and "What do you do for fill-in-the-blank?" And while I know it's unfair to act based on a half-dozen experiences, I refrain from asking many questions to unschoolers as in the past the defensiveness that accompanied the answer was too much to take. It felt like they thought I was challenging their philosophy instead of genuinely want to hear about what unschooling looks like (which is probably exactly how I've sounded when I do my sarcastic or defensive bit with non-homeschoolers). Naturally this is just my experience and in no way do I think all unschoolers would answer a question defensively, but it did make me realize that what people "do all day" is absolutely none of my business, whether I wonder it out of a curiosity aimed at improving and expanding my horizons, or plain and simple nosiness. And it also made me realize that many times, we hear things based on our intentions, not necessarily those of the person asking. I many interpret something as being asked with a "tinge of disgust," but that may not necessarily be the case. I know I never asked an unschooler about their day out of a sense of judgment, but maybe that has been their experience and they were geared up for it (much like I'm geared up to answer the "what about socialization" question).
And in reality, the question is impossible for anyone to answer - not just homeschoolers. What one does from day to day changes in small ways, large ways, based on emotions, physical healthy, extended family and friends, etc. And lastly, what one does all day isn't really anyone's business unless you make it their business. It's kind of like asking a couple that doesn't have children when they will start their family. I'm a proponent of openness and sharing, but there is a time and place for everything, and some things are better left unsaid. I personally don't enjoy hearing mothers of babies discuss diaper changing and bowel movements, napping schedules and breastfeeding - even if that's what they are "doing all day." I'm sure I did at one time, but that time has passed! So for me, unless I'm in a setting where questions are clearly welcome, I think I'll let the answers present themselves in a more organic way. If I genuinely want to expand my horizons, experiential answers are usually better for me anyhow.