Every other day?? That doesn't seem like nearly enough care for a dog! Even the most low key, laid back dogs I've pet-sat for needed to be let out at least twice a day, and fed once a day.
Also, will you still be paying your nanny her standard pay for the time you are away? If you are, I think a small thank you amount of around $25-50/week is probably sufficient.
If you're not paying her her usual pay...well you should. If I were your nanny and was told I wouldn't be making at least my standard pay for however long you will be away, AND I was still expected to take care of the dog, you can bet I'd be using that time to find a new job.