Here's my one rule for the sketch book: Bible verses or passages only. No doodles, no poetry, no Origami. (A blank page of paper holds so much possibility!)
I have found my quiet times to be so enriched by the creative, kinesthetic practice of writing out bits of Scripture. It makes each word soak into my mind in a way that simply reading doesn't do. Even more importantly, it's an act of worship. A verse may strike me with such poignancy that I can't help but copy it down and add some color and flourish, as if to say, Look, Father! Look at this beautiful thing You have said! What a magnificent Book You have written.