I do different things at different times. Last year, I devoted my brainpower to teaching an adult ed class once a week (the subject was Ecclesiastical Latin). This year, I am using a few minutes here and there to write poetry and essays about life with DS, and I'm getting my stuff published...I read zines published by at-home mothers for inspiration. When DS was first dx'ed with autism, I was reading books on neurology, psychology (including Mary Sheedy Kurcinka's book on spirited kids) and all kinds of conditions related to autism. That was some heavy stuff. When DS was a baby, I actually wore him to teach a weekly adult ed class on Biblical Greek until he was able to tolerate being left with DH (he had a serious issue with separation from birth).
There are so many things you can do with your kids for intellectual stimulation -- nursing homes always need volunteers to assist with various activities, such as reading or helping people at religious services, and they treat kids like royalty. Many special ed preschools and elementary programs need children who are developing normally to volunteer in the classroom, too. At my son's program for autistic kids, there are "Peer Pals" who visit class once a week for group activities so that everyone can learn appropriate social skills. The peer pals get very special attention from all of the adults (the adult child ratio is 1:2 in most autistic classrooms) and are treated like celebrities by the kids. Programs for blind and/or deaf children need this type of help, too. It is definitely a learning opportunity for the parents of the peer pals -- you'll learn more about brain development than you ever thought possible without attending med school!