NEVER EVER leave them alone in a locked house - they will develop behavioural problems.
You can do the following to stimulate your Yorkie:
Get them a few toys - balls (chewable), furry animals, ropes, chewees (bones made from animal hyde)
Make sure they have enough space to run
Plenty of grass - they like to run and dig
A stick or a ball for fetching - terriers naturally do fetching
Physical touch and praise from you
Never scream at the dog, they will fear you
