Kongs with peanut butter in them, frozen overnight. I even use some peanut butter to 'plug' the hole, so they have to get through that to get to the rest.

I have big dogs that love to chew, and have what we call the 'gobstoppers,' though that's not their actual brand. I think it may be bento balls or something of the nature. The treat is edible and replaceable, and my dogs love them. It generally keeps them entertained for an hour or so.

Also, puzzle toys! We have one from the new Toys 'R Us line that is a timed release ball that my Great Dane LOVES. It's like Pavlov's bell. The little 'releasing more treats' bell dings, and he'll wake from a dead sleep to go roll the ball around until the treats are all gone, then come lay back down. While not chewing, it keeps his brain and body moving to wear him out.

We also do regular rawhide a every other day or so. Compressed rolls last the longest for us.