I feel the same way. I pretty much decided I need to start homeschooling myself during this time. More of an unschooling approach, but making sure I intentionally schedule in the reading/book or research work into my week. It will mean cutting out computer/fun reading time, but I am really looking forward to it just the same.
I like organizing things, so I am heading out to the store to pick up a planner today. I will set aside certain nap times or an hour of nap times, and or evenings to work on different interests. I want to go into wildlife biology one day, so I ordered a college/grad level text book from Amazon and will spend part of the time working through that. This will improve my math, science and critical thinking skills. I already have a BA in Biology, so I have the core covered, though I don't really remember much since leaving school 4 years ago. I am hoping this will refresh my memory. I also want to pick up some historical fiction books, or some books on different cultures, since I have been interested in those topics recently.
I only have one toddler (17 months), so that makes things easier in some ways. I also know we won't be having more kids, and will be homeschooling, so I know there will be opportunities to continue my studies in the future. I think I would like to start doing some wildlife observations as well, which I can easily do with a toddler in tow. Hopefully it will be fun, but I guess if it isn't working we'll make adjustments.
I think for some staying home with the kids is not enough, actually probably for most. You need to find some interests that work the other parts of your brain as well. Whether it is subscribing to some informative magazines, Popular Mechanics, Popular Science, National Geographic, the New Yorker..., doing book work, reading as much as possible, or taking classes; I think we need to find a way to keep our brains engaged as well.
Hope that was helpful, I'm sure others have some different ideas.