Oh, I know about under-exercised dogs! It's been in the single digits or below lately and I have a 2.5 year old malinois. ick. But we do what we can. And you can create a toy motivated dog.
Clicker training is wonderful to stimulate your pup's mind and create a stronger bond between you and your dog. It's fun, doesn't take a lot of room and you can do it sitting in a chair. Right now, at dinner time, I'm working on Zest (my 3year old basenji) finding and retrieving my slippers. We also do nose targets, she taps my foot with her front paw, spin right, spin left, go to the mat, etc. Just look around on the internet for clicker training, there are LOTS of websites and useful stuff. Be sure to look up Karen Pryor's website. Enjoy, have fun with your pup.