What should you give a Weimaraner as a treat that is good for the dog?

Weimaraners are just the same as other dogs with regard to treats ;)

A treat to the dog is by definition something that he does not get in the normal way of things eg only at training etc, the dog will tell you what HE considers a treat.  Some suggestions:

Any fruit or vegetable bar grapes or onions.
Pieces of cooked meat (any type)
Cooked liver (especially with garlic) or liver cake (if the dog can support grain)
If you use pieces of frankfurter or cheese be careful as the first will have lots of salt and the last will be very fatty and some dogs cannot ingest a lot of it because they have problems with the milk protein (casein) or milk sugar (lactose).
Of course you can use toys as treats, it does not have to be edible so ball games or tuggers are useful too.

HTH

It depends where you are.  If you are in your house, doing some training, then ordinary kibble might be rewarding enough for the dog to want to work.

If you are outside somewhere, you might need something more tasty.  Maybe cheese, maybe hotdog sausages, maybe sandwich meats like ham or chicken slice.  There are some commercial treats called Coachies which are good in terms of ingredients, but they would work out pricey if you were just using those.