Have you tried Sprinkles (TM) with your dog yet? The idea is to throw small moist pieces of dog food/treats over a large area of grass without your dog seeing you doing this and then allowing them the time to seek out the Sprinkles in their own time. It is a great way of stimulating your dog mentally and can help to reduce stress and boredom.
We LOVE this video from Jane at The Dog House. She has made an indoor Sprinkles toy for her dogs. This "snuffle mat" is made by combining a holey rubber door mat combined with LOTS of bits of fleece. As you can see Dennis, Gizzy, Molly and Sandy seem to love it :)