The best way to keep you dog entertained taking your dog out for exercises, buying him some toys, also take him/her to a dog play date. You can also consider doggie daycare, getting him/her a recreational bone or take him out for a walk. Check out http://dogs.about.com/od/dogtraining/a/bust_boredom.htm for more tips.
here's one idea. i bought a hollow plastic ball especially designed with a hole for dog food. it's tricky getting the dog food out again. my dog enjoys nuzzling it around the floor and occasionally a treat falls out. :)
you could go outside and play fetch, or if that doesn't work, there's also just petting them, or going for walks. when other dogs come over they have a lot of fun. Toys are really useful, they keep you from having to constantly watch the dogs.