Have a schedule already written out on your "blah" days. Have something planned for each 1/2 hr, like help fold laundry, read a book w/o pictures, read a magazine, kid #1 picks out a puzzle and everyon has to do it, cook dinner, group yoga, sweep and vaccum the stairs etc.
Have a jar filled w/pieces of paper of things like projects or games you guys can do when boredome hits. I have a jar like that, and I have supplies in a bag for each activity, so all we do is reach into the jar, pull out an idea, and then I go find the baggie of all the things we need to do. (I got my ideas from a book I found at the used book store "365 things to do on a rainy day".)
Keep a list of places you can go on the spur of the moment. Write down the name, hours, phone number and any fees you have to pay for adults/kids. That way, if you have to get out of the house, at least you won't be scrambling around to think of a place to go. Try to avoid listing the mall or movie theaters. Save those for really really long days at home, like holidays and school cancellations.
Another thought is a list of people you and your kids can go visit, like an elderly woman in your neighborhood or in your church. Maybe you can even bake cookies and make some homemade cards for him/her.