There are rigid covers with perforations that are supposed to keep the debris out, but once we got our metal roof, we couldn't apply them to the gutters, if that's what we were going to do.
The other day at Home Depot, we saw these foam gutter guards, and bought some to try.

It's a coarse foam product, and seems to work fairly well, so we bought some more. It'd be too expensive to fill all the gutters, so Mr. Troutbend is going to cut them into sections cover just the holes for the downspouts. We'll just see how that works out.