We play hide and seeks with ourselves. One of us will hide and call out "come find meeeeee". The hider will be hiding with a treat. It is so cute to see the dog checking out all of the past hiding spots (behind the doors, bathrooms, closets, etc). Make it easy at first until he gets the hang of it. Our dog loves it and it wears him out with all of that thinking and sniffing.
We also work on a lot of tricks indoors because it doesn't require a lot of room since the dog is right there beside you. Try weaves, tween (the dog comes from behind in between your legs and sits to look up at you), roll over, spin etc. This also builds engagement!