It's something to have checked out, especially if your dog is normally a fairly active being and they just, all of a sudden, start to decline in how active that they are willing to be around the house. It may then be worth checking out. There are certain diseases and certain types of pain that can cause them to slow down.

You can work on their obedience as that alone often stimulates their minds. Toss the ball in the house, play tug of war, or give them their favorite toy or treat that you choose.