"As for pre-school, you already know I believe that a good home, with parents in tune with the kids, is the best pre-school. My obsession: letting kids develop their naturally long attention spans. Over and over and over when I'm out in public and observe parents interacting with children, I see the parents stifle or crush or end the attention/interest of young children. Adults in our consumer culture are the ones with the attention-span problem, not children. I believe in nurturing kids' interests and attention as much as possible. Two hours with a couple dumb blocks or a toy hammer or a little truck? Why not? Of course, it's not easy. So I've tried to be the quiet voice my daughters (I was a stay-at-home dad with them until they went to [Catholic elementary school]) and N. had to listen to--talking about things, verbalizing what's going on, reading things--most anything--aloud, using numbers in a calm way to build a sense of quantitative reasoning, and explaining as best I could whatever questions come up. I guess I try to model "thinking, learning humans" so that being curious or analytical or philosophical seems natural. I tried to use a variety of music every day; lunch, for sure, is a steady concert time, with lots of American traditional music but most anything else coming up so that musical literacy isn't just left to the marketplace (blues, folk songs, classical stuff especially for solo instruments to learn various sounds, jazz, big band, "world music" whatever). And if we have a banana, which we do every day, we talk about where they come from and their color and you get the point.
Outside, we worked on just being attentive [and] always trying to be engaged with our surroundings.
So, as to "not doing enough," I wouldn't worry. Personally I have a better feel for the everyday quotidian rather than the "big event" stuff like museums and events. Just being curious and communicative and open to their experiences and moods while at the same time being a kind of model of how to act and think--that to me does a big part of the job.
My method is pretty much seat-of-my-pants, as they used to say. I hope some of this might be helpful. You've got all the ingredients--both parents educated, interesting people, great kids, no huge tensions .... If you're having fun and trying to be there in the moment with them--avoiding praising and all that junk and instead just describing and being calm--you can't help but nurture in positive unforeseen beneficial ways.
It's a measure of how entrenched school (or a particular kind of reading-writing-arithmetic concept of school) has become in our culture that upper middle-class, highly educated parents worry about whether they can provide enough enrichment at home for a three-year-old. I don't know the data, but it seems anecdotally that a very large majority of children under age 5 with a parent not working outside the home nonetheless attends preschool. The "early-academics" model is so pervasive that it feels radical not to send your child to preschool. I'm not suggesting there is something wrong with sending your child to preschool or that there aren't benefits beyond academics to preschool. I was simply struck with how uncommon it seems today in my demographic to opt out of preschool.
Because it feels so radical to eschew preschool for one's child, the stakes seem very high. Hence our friend is fretting over what method to use, what program to follow. I remember this feeling! And I remember how liberating it was to befriend a veteran homeschooler whose calmness helped me see that the decision not to send my child to preschool was actually not a big deal. At no time is this case easier to make than when one's child is under five. Don't worry! It's going to be fine! And as Tim wrote to our friend, "Have fun!"
Bonus reading: Attention Span; Diverging Paths