Holy cow! I can't imagine not having enough work to fill my week. The thought is completely alien to my experience in the workplace. I assume y'all mean you have times of "feast and famine". That is, you work really hard some weeks . . . and other weeks have little going on. My husband's job is like that at times.
If I had extra time at work, first, I'd ask myself if I'm really doing all I'm supposed to do. I'd "work ahead" and prepare things that would help me when things get busy again. Then I'd look around and see if I could do anything that'd help me move up in the company -- you know, take on any extra jobs that're going undone, help other people who may be more busy than I am. I know that lots of people here claim to genuinely hate their jobs, but if you're taking the pay for a full week, you do have an obligation to put in a full week's work.
If no work existed and that just wasn't a possibility, I'd bring home projects with me and use that time. For example, right now I'm working on organizing and typing my recipes into a single book. I'd work on a couple things I'm making for Christmas presents. I'd work on buying/pricing items for the house we plan to build in a couple years (for example, I'd stalk light fixtures on ebay). I'd take care of my personal correspondence, or even this year's Christmas cards (which, if I had all of your free time, might be homemade). I'd plan out next year's vegetable garden. I'd plan next year's vacation. I'd organize my digital photographs and send them out to be printed, then I'd actually get them caught up into scrapbooks. I'd read.
If I had extra hours in my day, even if it were at work and I wasn't free to do literally anything I wanted, I could absolutely fill them with projects that interest me. Boredom wouldn't be an issue.