Have you considered hiking
I live in house with a small yard no garden. I take Wispa for a walk in the mornings before work and about 1 or 2 hours after work. At weekends I take him for a 40k hike in the countryside. This does us both some good and he loves it especially if we pass somewhere he can swim.
Sometimes a dog just needs a day out in its natural environement away from the city. If you are not convinced your doing enough then consider a bit of hiking at weekend. You may both enjoy it.