Training, just be patient and teach him new things. He may be a senior but he can help pick things up off the floor and hand them to you. There are several stimulating mental games you could do as well. Like one I play with Hemi is the old shell game or 3 card monte, I get 3 solo cups. put a treat under one and then mix them up, let him pick which one it is under. What about teaching him his colors, yes dogs can see colors, Hemi has learned 4 distinct colors. Basically go to like dollar general look for some cheap but completely identical things like cups, plates, child sandbox toys like bucket and shovel that are all identical in shape but different colors. Start with just 2 colors and two objects, but introduce more different objects of the same color later. It may take a few sessions to figure out what you are asking once he has 2 colors add a third. Lay them out, point to each one several times, and say the colors loud, This is Blue, This is Red. Then ask him to point out the red one. If he is correct treat and praise and reinforce saying yes that's the red one. If wrong say no thats blue, I want the red one and show them red. Its a lot of fun once they understand colors.
A good book with a lot of interesting ideas and brain games is Beyond Fetch by Caroline Coile
Beyond Fetch: Fun, Interactive Activities for You and Your Dog: D. Caroline Coile: 9780764517679: Amazon.com: Books
Oh and sometime its just good to be fun and silly with them. Seize the moments that happen. Like last night I was playing the piano, Hemi came over and was nudging my arm, I was like you want to play? I told him to put 2 paws up on the bench, which he jumped up on the bench in my lap. The piano bench is not that big but here we were. So I pushed the keys, he mashed the keys with his nose. We made a terrible racket for about 10 minutes doing this off and on but a tail was wagging and he was smiling the whole time. Then he got bored and got down and I could finish playing what I wanted while he slept beside me.