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.

You may want to avoid having those longer, strenuous walks, especially if your dog isn't used to that. If they've been conditioned for it their entire lives, stick with it, but I wouldn't go on any major hikes or walks that could cause them to have bad joint problems afterward. The same goes for jumping after a Frisbee where they may be more prone to getting injured. I would avoid that.

It's about having them interact with you. Many times, it's getting them up and moving and going on a walk at a slow pace where they can still get up and move and enjoy that time. If they love to play fetch, do that. They may not be able to go the longer distances anymore, but having them up and active with you is good to have them doing well in their later years.